I just remain calm. There's no use in yelling at someone who's head's on fire. It usually pisses them off more when their temper's flaring, and I just sit there and communicate in a calm, cool manner. Then I get "HOW COME YOU'RE NOT MAD??!!?" I've never seen the point of raising my voice.
If I find myself actually getting pissed off, I'll say "We'll talk about this later", and go do something else.