Arrival: Frist thing, greet the host. If you came with a guest (spouse, girlfriend, bro) make sure you introduce them as well. If this is a birthday party for a child, take a minute to wish the kid a happy birthday when you first arrive.
What did you bring?: If you brought a gift, drink, or dish, bring that straight to the host. If you were asked to bring something, you better bring it. The host is counting on you. Men fulfill their obligations.
Helping: Offer your assistance to the host "What can I do to help?" They will most likely not need your help but will appreciate the gesture.
Drinking: It's okay to pop a beer open (or two) at a kids party, any more than that and you become "That Guy." If this is a more casual get together, just don't get too intoxicated.
Clean up: Unless you have to leave early, don't bounce out as soon as the cleanup starts. Offer to help out again with pickup up. It doesn't take long and you'd appreciate that if you were the host.