Anger is an emotion that arises as a result of your inability to control something. Give it a try. Think about times in your past where you've gotten angry or agitated at something or someone.
There is actually very little that we have control over. Realize this and things will stop getting to you so much. Life would be pretty boring if we always got what we wanted and everything just worked out. With that in mind, here are some tips anybody can use whenever a situation arises.
1. Expect anger. It's a normal emotion that will crop up every once in a while and should be expected.
2. Visualize exhaling anger. When angry, breathe in deeply and imagine that breath is soaking up the anger. Then breathe out visualizing the anger leaving your body.
3. Count to 10. Create a gap between what's causing you to be angry and your response. Slowly counting to 10 (or perhaps a higher number ;) ) can refocus your attention on something else. That redirection of your attention can create enough of a gap for you to get composure. It's all about responding - NOT reacting.
4. Apologize and mean it. Apology should come from the heart and be truly honest.
5. Forgive yourself. Don't beat yourself up too much. People get angry and it's a natural response.
Does it matter? You're angry about something - does that something matter? Are you going to remember this 5 years from now? How much time are you willing to give up to the current feeling of anger? 5 minutes, 5 days, 5 years?
6. Move on. Live in the present moment. If you hang onto the anger, you'll be living in the past. Worse yet, you'll also be giving up your future by spending more time than you want to focusing on what triggered your anger. The faster you can move on, the faster you can get on with life.
7. Get sleep. The next time you're angry ask yourself if you got enough sleep the night before. If the answer is no, then perhaps you're not really angry at what just happened but are instead grumpy from a lack of sleep. A full night's rest will put you in a good frame of mind and you will likely be less angry than otherwise.
For something more substantial try reading American Psychological Association - Controlling Anger Before It Controls You.