Always have a drink of water in your hand.. even alongside an alcoholic one! Water will help fill you up so you will indulge less..
Also, eat something healthy (and filling) before you go - NEVER go to a cocktail party hungry, you will eat a million spring rolls and things! Also, try to keep track of what you eat - try not to get into double figures with the canapes (it's harder than it sounds..)
Have fun, DANCE (burns calories!)!!!!!