How to Respond When Someone Flips You Off in Traffic

How should you respond to someone who flips you off on the road? This is a question I asked my friends and family after someone flipped me off in downtown Boise the other day. I was driving down Broadway, going with the flow of traffic, and yet the person behind me refused to lay off my bumper. When he finally got to his turn, he rolled down his window and flipped me off. I couldn't help but scratch my head as to why, and my knee jerk reaction was to flip him off right back. But, I had my 3 year old in the car and I have to be the bigger person now. (Sigh! haha!)
Anyway, the responses I got from friends and family where both comedic and helpful. This is actually the first time I've been flipped off since living in Boise again... and let me tell you, it's a daily occurrence in other cities I've lived. The advice I received where as follows:
1. Always have an alternate route. If someone starts being crazy on the road, have an exit plan.
2. Smile and Wave
3. Absolutely nothing
4. Whine about it on my Facebook status

What do YOU do when someone is being aggressive on the road?