One of the problems with pre-dinner snacks and party food is that they're always served when you're at your hungriest, and it's diffcult not to gorge yourself on unsatisfying 'non-foods' full of empty calories purely because the body needs sustenance!
Stave off the urge to snack by having a healthy afternoon snack (an apple, some prunes or apricots and a pot of natural yoghurt), or have a filling snack half an hour before you head to the party (raw carrots, tomatoes and cucumber, a yoghurt and a big glass of water). The fibre in the veg and protein from the yoghurt will fill you up instantly, and the water will quench your thirst so you won't mistake it for hunger.
