This one is both obvious and underutilized. When we are angry, we are physiologically aroused. Our heart-rate increases, our muscles tense, our breathing increases, etc.
One way to deal with all that is to take long, slow, deep breaths, using the diaphragm rather than the just the chest. Deep breathing is one of the best ways for people to relax, especially in a tense moment. They key is to remember to do it.