First, I try to eat BEFORE I go to the party so I'm not very hungry, even if it's a dinner party. Then I try to look over everything that is available to eat before I get anything. I pick and choose.
My philosophy is that if I'm going to eat something sweet, I try my best to make it something very special, and even then, I limit the portion size and how often I eat sweets.
Often, there will be veggie trays or fruit trays, which are PERFECT for grazing. If there are dips with them, I take a LITTLE dip.
Finally, make sure to schedule workout time around the party. If I know that I worked out the day before and the day of, I don't get nearly as concerned. You can also include a workout the day after the party.
Enjoy! Good luck! It can be done in the midst of a healthy lifestyle!!