1. Prepare!

Psychologically - Parties shouldn't be the only place where you see your favorite foods and allow yourself to enjoy them! If you ditch the dieting mentality and allow yourself to savor small amounts of the foods you love on a regular basis, you'll be much less prone to overindulge. (Continued tips on how to do so will appear periodically on this site.)
Physically - Don't "go" hungry. Be sure you've had your regular, healthy and satisfying meals prior to the party (i.e. breakfast prior to a luncheon party, breakfast and lunch prior to a dinner party).
Thirst can often be confused with hunger so drink at least one bottle of water just prior to the party and chose to be careful about the beverages you drink once you are there. If you are drinking three beverages have only one glass of anything caloric - no more than one alcoholic beverage (if alcohol is allowed for you), or soda or juice. Your other two glasses of beverages should be either water or another non-caloric option.
2. Be Picky!
At the party select several of your favorite foods to eat. Sit and savor them as you disconnect from any concerns about your surroundings or life away from the party. Allow yourself to be there in the moment as you notice the sight, scent, taste and texture of the treats you chose - reminding yourself that you can savor a small amount like this whenever you chose, so you need not necessarilly finish, nor eat past fullness.
3. Go for the Positives
Many people have party "phobias". One of the best pieces of advice for this problem is to focus on finding a way to sincerely compliment at least three people at the party on something other than their appearance. Why does this work? You'll be forced to focus not on yourself and what others are thinking about you, but on getting to really know those around you, who and what interests them, and what they love to do.
Most importantly, as I often advise my clients —
have fun!