This might sound like a difficult concept but it's really not. All it is is taking those few extra seconds when you start to get angry, to have a think about what getting angry in this situation might do to it. The answer is usually "make it worse" and even if not, don't you want to be happy?
There are a million things in life that can make us angry if we let them, but most of them we can't change anyway. So, there really is no point in getting angry. Choose your own happiness by not letting it get to you. In every potential situation, ask yourself what is most important and you'll almost always find that the answer is your own happiness (which you are more in control of than you often think).