I found this article on Borderstan on how one should behave at a Happy Hour with Coworkers but I believe it also applies to friends. I have to say, I agree with most of the rules:
Eat before you go out. If someone starts passing around strong drinks or shots, You want to make sure you can handle then and or drive home.
Do what you want to do? There is a lot of peer pressure at Happy Hours to keep the fun going. But don’t let friends talk you into that next bar.
Talk about what you want – feel free to leave awkward conversations that you don’t enjoy (unless its your boss). I use the I need another drink or I’m running to the bathroom. If you don’t need either, just do a loop of the bar.
Meet new people. The most important thing you can do at a happy hour is network, with coworkers, with new friends, with old friends, or with potential paramours. Take this time to find out more about people so that when you need a favor you can warm them up by asking about Hoyas Basketball!