Do have a designated driver if you're going to drink!
Take cigars to hand out, and take something to hand out that you would smoke, no dog rockets! Every herf I have attended you end up bringing more cigars home than what you took!
If the herf is at a lounge, resturant, etc. make sure you don't stiff the wait staff on paying the bill or leaving a tip (you would not believe the number of times I've seen this happen).
And the #1 thing to remember at a herf is........................................
HAVE FUN!!!!!!!!!!!!!!!