They key for me is tiny servings of everything I love...
One small spoonful of potato salad, one tiny slice of ham, one tiny serving of...etc. etc. etc.
The key for me during holidays is to try a tiny bit of everything, and like glory said DO NOT TAKE ANYTHING HOME!! It is too tempting when it is in your refrigerator.
Also, definitely listen to your body..if you are full, and haven't gotten to the chocolate cake...it's okay. Don't have the chocolate cake. It's not like that's the last chocolate cake you are ever gonna see