Believe me, I have a hard time taking my own advice. But I've had enough rage-inducing encounters to conclude these things:

• Somebody who does something outrageous in traffic is probably already stressed out and on edge. Bad time to push them.
• It's easier to cool down after such an affront if you don't escalate it. I may be mad for a few seconds if someone does something stupid, but if I turn it into a shouting/gesturing match, I fume for a long time.
• Learn from it. Chances are that if one motorist did something stupid at a certain spot, others will. Take a different approach there, or find a way around it.
• Remind yourself at the start of each ride that you really enjoy it, and you're going to have fun. It may help you roll with the punches.