1. If one was unable, (or forgot) to daven Shachris until Chatzos, one should not daven Shachris that day, since the time for Mincha has already arrived. Rather, he should daven two Shmona Esrei's at Mincha, (the first for Mincha and the second as tashlumin for Shachris). {However, B'dieved, if he davened Shachris up to one half hour after Chatzos he is Yotzeh}.
Shulcha Aruch 89:1 with Mishna Brurah
 
2. If one purposely neglected to daven Shachris, one may not make it up with two Shmona Esrei's at Mincha. Shulcha Aruch 89:1 with Mishna Brurah
 
3. Before Davening Shachris one may not say Sholom Aleichem to another person, even his Rebbi or Father, because Sholom is the name of Hashem and it is not fitting to honor a person before having davened to Hashem. However, it is permitted to respond Aleichem Sholom if someone greeted you first. (If one already said Brochos, one may say Sholom Aleichem, even before davening). Shulchan Aruch 89:2 with Mishna Brurah 
 
4. One may greet another person with Good Morning, or any other greeting that does not contain the name of Hashem, even before davening.
 Shulchan Aruch 89:2 with Mishna Brurah
 
5. In the morning one may not take care of his business or personal needs, or travel before davening Shmona Esrei. However, Bshaas H'dchak, for example if one needs to catch a plane, one may travel before davening and daven on the way if he will be able to stand for Shmona Esrei. Shulchan Aruch 89:3 with Mishna Brurah
 
6. Notwithstanding the above, on Erev Shabbos & Yom Tov it is permitted to purchase Shabbos/Yom Tov needs before davening, as these are considered Cheftzai Shomayim, and not personal needs. Mishna Brurah 36 @ SA 89:7
 
7. L'chatchilah one may not eat before davening Shachris (weekdays and Shabbos), but it is permitted to drink to help one's Kavannah in tefilah. Shulchan Aruch 89:3 with Mishna Brurah
 
8. However, if one needs to eat for health reasons, or if he will be unable to concentrate his tefillah without having eaten first, or if he is particularly hungry, it is permitted to eat before Shachris; but one should at least be Mekabel Ol Malchus Shomayim and say She'ma Yisroel before eating. Shulchan Aruch 89:3-4 with Mishna Brurah 
 
9. One may not daven Sh'mona Esrei on any type of platform, or bench, etc. for two reasons;
a) because he is afraid of falling- this will distract his kavannah, and
b) because of the posuk in tehilim; "Mi'mamakim Korosicha Hashem" - from the depths I called to hashem. Shulchan Aruch with Mishna Brurah 90:1
 
10. However, it is permitted to daven on a raised platform such as a bimah that has mechitzos, as this addresses both concerns; one is not afraid of falling, and it is considered it's own r'shus - not a raised area of the main r'shus. Shulchan Aruch with Mishna Brurah 90:2
 
11. L'chatchila one should not daven Sh'mona Esrei in a room without windows; so that if he loses concentration he can look up to shomayim and regain his Kavannah. One also should not daven outdoors in a place without any mechitzos (such as an open field or street), rather he should daven in a place that has a level of tznuis; as this helps bring out fear of the King and breaks his heart - pre-requisites for proper tefillah. Shulchan Aruch with Mishna Brurah 90:4-5 (see also 95:2)
 
12. If one must choose between davening outdoors in an open area, or davening indoors in a room without windows, it is preferable to daven in the room without windows (since the Gemara calls one who davens outdoors in an open area a "chatzuf" - an arrogant person). Shulchan Aruch with Mishna Brurah 90:5
  
13. One should always try to daven in a shul with a minyan because "Hashem is not disgusted by the tefillah of a tzibbur". If one is an Ones and is unable to go to shul to he should at least try to daven Sh'mona Esrei at the same time as the tzibbur. Shulchan Aruch with Mishna Brurah 90:9
 
14. If one was an Oines and missed the minyan, he should nevertheless daven B'yechidus in a shul, because it is a Mokom K'vuah for kedusha and his tefillah will be more acceptable in shomayim. Shulchan Aruch with Mishna Brurah 90:9
 
15. When one is davening in a shul it is not permitted to rush his tefillah and daven Sh'mona Esrei before the tzibbur, because by doing so one is "mevazeh" the tzibbur. If the tzibbur is saying extra piyuttim one should say the extra piyuttim along with the tzibbur, and not be a Poiresh Min Hatzibbur - even to learn Torah. An exception is if the tzibbur is late and will miss z'man tefillah - in that circumstance one may hurry his tefillah ahead of the tzibbur. Shulchan Aruch with Mishna Brurah 90:10
 
16. If one sees that he cannot wait to daven Sh'mona Esrei with the tzibbur because he must travel (ie. catch a flight) then it is preferable for him to leave shul, and daven B'yechidus at home, rather than davening in shul ahead of the tzibbur. Shulchan Aruch with Mishna Brurah 90:10 (Bach)
 
17. One who does not daven in shul even though there is a shul in the city causes that he and his children will experience Golus. On the other hand, one who arrives early to shul for tefillos will be rewarded with Arichas Yomim. Shulchan Aruch with Mishna Brurah 90:11
 
18. It is a Mitzvah to run the last few steps into shul (or when carrying Tallis/Teffillin and it is obvious that you are heading to shul - even the entire way), but one must be very careful not to run in shul or when leaving shul. One must also be careful not to schmooze on the way to shul or upon having arrived in shul and thereby miss Kaddish or other portions of the tefillah.
Shulchan Aruch with Mishna Brurah 90:12
 
Addendum
18.a The Mitzvah to run to shul is applicable even on Shabbos when it is normally an issur to run.
 
19. One should try to arrive early in shul before davening in order to be counted amongst the first 10 in the minyan. Shulchan Aruch with Mishna Brurah 90:14
 
20. One who is at home is required to travel a "mil" (a measure of time generally accepted to be 18 minutes) to daven Shachris, Mincha or Maariv with a Minyan. {When traveling, if in the direction one is headed anyway he can find a minyan - and by doing so he will not miss zman Krias Sh'ma or zman tefillah - he is required to travel even up to 4 mil (72 minutes) to daven with a minyan}. Shulchan Aruch with Mishna Brurah 90:16
 
21. One should establish a steady shul in which he davens, and in that shul, a Mokom Kovuah because this helps a person to concentrate on his tefillah. We learn this from Avraham Avinu who had a Mokom Kovuah for tefillah. {One should also have a Mokom Kovuah for tefillah in his own house for instances when he is unable to daven in shul.}  Shulchan Aruch with Mishna Brurah 90:19
 
22. If one comes to shul and finds someone sitting in his seat, it is not necessary to ask the person to move, because anywhere within 4 Amos (appx. 8 feet) is considered to be his Mokom Kovuah with regard to this halacha. Shulchan Aruch with Mishna Brurah 90:19
 
23. One should always daven Sh'mona Esrei from a siddur and be careful not to move his eyes from the siddur to avoid losing concentration on his tefillah. Shulchan Aruch with Mishna Brurah 90:21
 
24. It is not permissible to daven facing a picture or photo of a person, even if one closes his eyes while davening, because it appears as if he is bowing to the person rather than to Hashem. Therefore, when davening in a home (ie. during a shiva r"l) any pictures or photos on the mizrach wall should be covered. Shulchan Aruch with Mishna Brurah 90:23
  
25. L'chatchila one should not daven Sh'mona Esrei directly in front of his Rebbe or his father because it looks like he is arrogant and considers himself on the same status. Shulchan Aruch with Mishna Brurah 90:24

 

25a. One should also not daven behind his Rebbe or his father because of two additional reasons; a) it appears as if he is bowing to the Rebbe/father and b) he may cause the Rebbe/father to delay stepping back after completing Sh'mona Esrei if he is still davening. Shulchan Aruch with Mishna Brurah 90:24
 
26. In a shul where each person has a Mokom Kovuah one need not be concerned with the above mentioned problem of appearance of arrogance or bowing, since everyone knows that these are the established seats for the Rebbe and the talmid. Nevertheless, L'chatchila one should not establish his Mokom Kovuah behind his Rebbe because of the third problem of causing the Rebbe to delay stepping out, which is still applicable.
Shulchan Aruch with Mishna Brurah and Biur Halacha 90:24
 
27. One may not daven Krias Sh'ma or Sh'mona Esrei, or learn Torah in the presence of human waste, dirty childrens diapers, a foul smell, a dead body, or an uncovered ervah.
Shulchan Aruch with Mishna Brurah 90:25
 
28. If a child accidentally soils himself from behind in shul in middle of Sh'mona Esrei, those within 4 Amos must stop their tefillah until it is removed or covered. If those within 4 Amos do not realize, one should step out of his Sh'mona Esrei and even speak if he has to, to let them know that it is an issur Dioraysah to continue their tefillah. {If it is covered, but still smells, one may not continue his tefillah and should step out and move more than 4 Amos away}. If the waste is only urine, one may not speak in the middle of his Sh'mona Esrei, because urine is only a problem m'dirabbonon. Shulchan Aruch with Mishna Brurah 90:26
 
29. One may not daven while any part of his body is uncovered which normally would be covered, because one must daven as if he is standing before a King with fear and awe. B'dieved if one did not have clothing to cover his entire body, as long as below the waist was covered he was Yotze and need not daven again.
Shulchan Aruch with Mishna Brurah 91:1
 
30. Additionally, before davening one should make a separation at his waist between his heart and the Mokom H'ervah to fulfill the posuk of "Hikon Likrosecho" ; prepare yourself to greet Hashem. For this reason many have the minhag to wear a gartel for davening. Others however, rely on the psak of the Bais Yosef that the necessity to wear a gartel is only for one who is accustomed to wear a gartel all day, and/or rely on the waistband of one's undergarments which is also a separation between the heart and the area below the waist.
Shulchan Aruch with Mishna Brurah 90:2
 
31. One may not make a brocha, mention the name of Hashem in a tefillah, or even enter a shul, with an uncovered head. Any type of covering suffices for this purpose, except that placing one's own hand on his head does not help, since his hand and head are one body. On the other hand, someone else's hand on his head does help. {Also, if one pulls his shirt sleeve over his own hand; this also suffices}. Shulchan Aruch with Mishna Brurah 91:3-4
 
32. One may not daven in any state of dress or undress which would not be acceptable in society when meeting with prominent people; This specifics of this halacha depend on the accepted norms of each country and society. Examples mentioned by the Mishna Brurah as being unacceptable are: wearing -sandals, any type of shoes which bare one's feet, pajamas, underwear, or not wearing a hat. Shulchan Aruch with Mishna Brurah 91:5
 
33. If one feels the need to relieve himself he may not daven, say Krias Sh'ma, learn Torah or even say a Brocha. If one did daven in that state, if he estimates that he would not have been able to hold it in for 72 minutes he is required to repeat his tefillah. {However, one who is in middle of giving a shiur is not required to stop in middle because of the Kovod of those listening to the shiur). Shulchan Aruch with Mishna Brurah 92:1
 
34. Before one begins his tefillah he should check to make sure that he does not have to relieve himself. This is true even if by spending time on this he will miss tefillah b'zibur. However, if this will cause him to miss zman tefillah completely he may daven, provided that he estimates that he can wait 72 minutes. Shulchan Aruch with Mishna Brurah 92:1
 
35. If one did not feel the need to go to the bathroom before beginning Krias Shma or Sh'mona Esrei, and then feels an urge during his tefillah; if he will be able to complete the tefillah without going to the bathroom, he may do so. {However, following the completion of the 3 parshiyos of Shma or Sh'mona Esrei, as the case may be, he must go to the bathroom before beginning V'yatziv and before Kedusha, respectively}. Shulchan Aruch with Mishna Brurah 92:2
 
36. If one feels an urge to go to the bathroom during Psukeiu Dzimrah, he may not say Borchu before going to the bathroom. Shulchan Aruch with Mishna Brurah 92:2
 
37. Before saying Krias Shma or before beginning Shachris, Mincha or Maariv one must wash his hands with water. This chiyuv is so essential, that the Rambam paskens that for one who failed to wash - his tefillah was not a tefillah; although we pasken l'halacha that he need not repeat that tefillah. Shulchan Aruch with Mishna Brurah 92:4
 
38. If one does not have water available and by going to wash his hands he will miss zman tefillah (or zman Krias Sh'ma), or tefillah b'zibur, he should rub his hands on anything available that will clean his hands as best as possible. Shulchan Aruch with Mishna Brurah 92:4
 
39. One should not touch any part of his body that is normally covered, scratch his scalp, touch the inside of his ears or nose while davening or learning. If one did; L'chatchila he must wash his hands, and B'dieved he may rub his hands on anything available to clean them. {Blowing one's nose with a tissue or handkerchief is not a problem}. Shulchan Aruch with Mishna Brurah 92:7
 
40. It is very good to give tzedakah before tefillah based on a posuk in tehillim (p.17) "Ani B'zedek Echezeh Ponecha" - loosely translated as "With tzedakah I will see your face". Many people also have the minhag to give tzedakah upon reciting the words "Vh'osher Vh'akovod Milfonecho...." in Vayevorech Dovid. The gabbai should be careful not to collect tzedakah during Krias Hatorah as this distracts people from properly answering the Borchu of the oleh, and listening to Krias Hatorah. Shulchan Aruch with Mishna Brurah 92:10
 
41. At a minimum, upon entering shul one should at least wait a few short moments before beginning his tefillah to allow his mind to concentrate and focus on the fact that he is about to begin to speak with Hashem, King of all Kings. Each moment spent simply sitting in a shul before and after tefillah is a mitzvah. Shulchan Aruch with Mishna Brurah 93:1
 
42. One should begin his tefillah with thoughts of fear of and humility to Hashem. If one finds it difficult to do so, he should at least bring himself to thoughts of Simcha shel Mitzvah, and being thankful for the opportunity to speak with and daven to Hashem. Shulchan Aruch with Mishna Brurah 93:2-3
 
43. We learn from pesukim in Tanach that one should always daven towards Eretz Yisroel, towards Yerushalayim, and towards the mokom hamikdosh. In the US and Europe, this means that we face Mizrach - East, but in other parts of the world it may mean facing West. Shulchan Aruch with Mishna Brurah 94:1
 
44. If for some reason one is unable to face in the proper direction, he should at least turn his face in the right direction. If one is davening in a shul which faces in another direction (for example South), he should daven in the same direction as the tzibbur, but turn his face towards East (or Eretz Yisroel). If one is unable to discern the proper direction, he should be mechaven his heart towards his father in heaven. Shulchan Aruch with Mishna Brurah 94:2-3
 
45. If one is traveling in a car, bus, boat or airplane (and needs to daven while traveling to avoid missing Zman Tefillah); if he is able to stand up for Sh'mona Esrei without being in fear of falling, he should try to stand up. If one is afraid that he may fall, he should nevertheless at least try to stand for the brachos where one bends and bows. Shulchan Aruch with Mishna Brurah 94:4-5
 
46. If standing up will be an actual Sakanah of falling and getting hurt, not simply a fear of falling, then it is preferable for one to remain seated. Shulchan Aruch with Mishna Brurah 94:4-5
 
47. If one is sick (or old) and unable to stand up for davening, he should daven even while lying down. If he is able to stand up at least for the parts where one bows, he should do so.
Shulchan Aruch with Mishna Brurah 94:6 

 
48. Of one is too sick (or old) to daven at all, he should at least try to think in his mind about the tefillah. If one is unable to even do that, he should at least think about the first posuk of Krias Sh'ma which one can concentrate on quickly. Shulchan Aruch with Mishna Brurah 94:6
 
49. One may not lean on, or against anything while davening Sh'mona Esrei or at any other time during tefillah when one is required to stand, (e.g., when the Sefer Torah is being brought to the Bima) because leaning is a lack of fear of, and honor to Hashem. Shulchan Aruch with Mishna Brurah 94:8
 
50. If one is sick and therefore unable to stand; if he is sure that by leaning against something he will still be able to have Kavannah he should stand and lean. Otherwise, it is better to sit for Sh'mona Esrei. Shulchan Aruch with Mishna Brurah 94:8

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Of the 613 Mitzvos, there are Six Constant Mitzvos. Rather than requiring the performance of a certain action, these mitzvos are a state of being, of living with the reality of God's existence. The Six Constant Mitzvos are:   

 * Believe there is a God.
* Don't believe in other gods.

* Believe God is one.

* Love God.

* Fear God.

* Don't be misled by your heart and eyes.