419. Even for those who keep the second minhag of Sefirah and therefore do not take a haircut after Lag Ba'omer; one is permitted to take a haircut during the "Shloshes Yimay Hagbalah" - the three days before Shavuos. Shulchan Aruch w/Mishnah Brurah 493:3
 
420. On Shavuos at night the minhag is to wait until it is night (Tzeis Hakochavim) to daven Maariv in order to have 49 complete days. Shulchan Aruch w/Mishnah Brurah 494:1
 
421. Likewise, it is preferable for women to wait until it is night to light candles and say Shehechiyanu on Shavuos night.

Luach R' Yechiel Michel Tukatchinsky,  Piskei Tshuvos 494:2
 
422. There is a minhag (brought in The Zohar) to stay up the night of Shavuos and learn Torah. This is based on the Midrash which says that Klal Yisroel slept the night before Matan Torah and Hashem had to awaken them to receive the Torah. Therefore we stay up all night to rectify their oversight. Shulchan Aruch with Mishnah Brurah 494:1
 
423. The Ari'zal writes that one who does not sleep the night of Shavuos and toils in Torah is assured to live out the year and no damage will occur to him.
Shulchan Aruch with Mishnah Brurah 494:1
 
424. There is a compilation of torah learning known as "Tikkun Leyl Shavuos" which has The Zohar and the writings of The Ari'zal as it's source, and many have the minhag to say/learn this on Shavuos night. Kaf Hachaim 106:7, Lev Dovid L'Hachidah
 
425. Generally women should not say the "Tikkun Lely Shavuos". However, some say that a woman who counted all the days of Sefiras Ha'omer may learn the portions of the Tikkun which are from Tanach. S"U Rav Poalim1SY:9, Kaf Hachaim 106:8
 
426. One who stayed awake all night of Shavuos should immediately, at Alos Hashachar wash one's hands for Netilas Yadayim without a beracha, and listen to Birchas Hatorah from someone who slept before one continues to learn.
Shulchan Aruch 4:14, 47:12MB28
 
427. Before Shachris one should use the restroom, wash his hands and then make the berachos of "al netilas yadayim" and "asher yotzar".
Shulchan Aruch w/Mishnah Brurah 594:1
 
428. One who stayed awake all night should not make a beracha on his tzitzis. Rather, he should make the beracha on his Tallis Gadol and have in mind to be yotze for his tzitzis with the same beracha. Shulchan Aruch w/Mishnah Brurah 494:1
 
429. One who stayed awake all night should listen to the berachos of "Elokai Neshama" and "Hamaveir Shayna Mayeinoi" from someone who slept during the night.
Shulchan Aruch w/Mishnah Brurah 46:MB24
 
430. One should refrain from marital obligations on the night of Shavuos unless it is the Leyl Tevilah or if one was not yet yotze the mitzvah of "Pru U'Revu" (one boy and one girl).
Shulchan Aruch w/Mishnah Brurah 494:1:MB1, 240:MB7, see Biur Halacha
 
431. One who goes to sleep after Alos Hashachar does not say the beracha of Hamapil.
Shulchan Aruch w/Mishnah Brurah 239:1MB8, Piskei Tshuvos 494:5
 
432. There is a well known minhag to bring flowers into the shul and home as a remembrance that on Shavuos we are judged on "Peiros Ha'ilon" - fruits. However, one should not cut off branches from fruit bearing trees for this purpose inasmuch as some poskim do not permit this because of the Torah prohibition of "Ba'al Tashchis".
Shulchan Aruch w/Mishnah Brurah 494:3, Piskei Tshuvos 494:10
 
433. Those who follow the minhag of the Gr'a (Vilna Gaon) do not place flowers in the shul. The Gr'a ended the minhag because it had become the custom of non-jews to decorate their places of worship with flowers.

Shulchan Aruch w/Mishnah Brurah 494:3, Piskei Tshuvos 494:10 
 
434. There is a not so well known minhag to bring good smelling grasses into shul to be passed around for people to make a beracha and smell the grass in remembrance of the Simcha of Mattan Torah where Har Sinai was surrounded by grass.
Shulchan Aruch w/Mishnah Brurah 494:3
 
435. When practicing this minhag one should not pass the grass during davening between Boruch She'amar and kaddish after Sh'mona Esrei because one is not permitted to interrupt davening at that time to make the beracha.
Shulchan Aruch w/Mishnah Brurah 494:3
 
436. Many have the minhag to eat a milchige meal (and milk and honey) on Shavuos. Some eat milchig at night and fleishig at the day meal, while others eat milchig, bentch, and then wash again, and eat fleishig to be yotze Simchas Yom Tov with meat.
Shulchan Aruch w/Mishnah Brurah 494:3
 
437. If one eats milchig but not "hard cheese" (which requires one to wait six hours before eating meat) one need not bentch and wash again. However, one should wash his /her mouth well, eat something hard like bread and the tablecloth should be changed. (Other poskim require one to bentch).
Shulchan Aruch w/Mishnah Brurah 494:3, Piskei Tshuvos 494:12
 
438. One should be careful not to use leftover challah from a milchig meal at a fleishig meal. (This is applicable all year as well). Rather, one should use new challahs for each meal. This also fulfills the minhag of rememberance of the Shtei Halachem (Two Loaves) brought on Shavuos in the Beis Hamikdash. Shulchan Aruch w/Mishnah Brurah 494:3
 
439. From Rosh Chodesh Sivan until the 8th of Sivan (Isru Chag) we do not say Tachnun. Some do not say Tachnun until the 12th of Sivan (inclusive), and in the Diaspora until the 13th of Sivan (inclusive). Shulchan Aruch w/Mishnah Brurah 494:3, Piskei Tshuvos 494:9
 
440. We read Megillas Rus on Shavuos because it was written to document the yichus of Dovid Hamelech and Dovid Hamelech was born and niftar on Shavuos.
Shulchan Aruch w/Mishnah Brurah 490:9, see also Shaarei Tshuvah 494:3
 
441. The Ollelos Ephraim says that one who dedicates a new Sefer Torah to a shul on Shavuos is considered as if he "brought a Korbon Mincha to Hashem in its right time".
Shaarei Tshuvah 494:3