If you're entertaining for a small- to medium-sized group, a self-serve bar is the key to a stress-free party.
Tip #1 : Get out of the kitchen... I like to set the bar in a room other than the kitchen - I find that guests like to crowd into the kitchen and setting the bar in the family room helps them to circulate around the house.
Tip #2 : Help Yourself... Make sure you have all of the tools you need for guests to help themselves. I use one type of glassware for wine, beer and water. You could also include a pitcher of a pre-made holiday cocktail, if you're feeling festive.