A cocktail party is not dinner. If you show up ravenous, you'll be disappointed. If you're not disappointed, then you've humiliated yourself by inhaling all the shrimp and downing a wedge of cheese as big as a doorstop. There's a reason for those small plates -- you're supposed to eat small portions and leave some rumaki for the other guests.
No double-dipping. Whether you go for the heart-healthy raw veggies or the carb- and fat-laden chips, plunge into the accompanying dip or salsa just once. After you've taken one bite, you can't go back again. Unless, of course, you've spooned some sauce onto your plate. Then, you can dip yourself silly.
Neatness counts. Always have a napkin handy. If you have just one hand free, it's OK to take an hors d'oeuvre from the tray and pop it directly into your mouth, as long as you wipe your fingers afterward. It's a good idea for you to keep your right hand free, so you can shake hands with other guests -- cocktail parties are excellent mingling and networking opportunities. That's another good reason for that napkin. You don't score too many points if you share your cocktail sauce with the person whose hand you've just shaken.
Don't be a juggler. If at all possible, put your glass down if you're using a cocktail plate. Some people have a marvelous talent for holding a glass and a plate in one hand, while feeding themselves with the other. Very few people have that talent. Watching an olive roll off your plate and across the floor while trying to balance the plate and your glass can be mighty embarrassing. As for that olive -- hold it in your fingers, take several small bites until only the pit is left, and deposit the pit on the side of your plate or in a napkin. Don't put the whole thing in your mouth, gnaw at it, and then spit the pit out.
Think while you drink. They're called cocktail parties, but chances are beer and wine will also be available. Drink beer out of a glass, not the bottle or the can. Hold your wine glass by the stem -- especially if you're drinking white wine. That keeps your body heat from taking the chill off the wine. And sip -- don't gulp -- wine. It lasts longer and tastes better that way. Whether you're drinking beer, wine, or an actual cocktail, take it easy. It's much better to look back on a party and remember all the holiday frivolities and foibles than to wake up the next day draped with tinsel from who knows where.
Have fun. The holidays come around just once a year, so socialize, hobnob, and make merry (moderately).