The number one thing to do is go with a plan in mind. Stop pretending you do not know what your M. O. (pattern) is at such events. You have had that "V-8 moment" after overindulging at a party far too many times to fool yourself anymore. Think ahead and decide what your strategy is going to be as far as eating (and even drinking). Then, stick to it! You may not do everything you planned to at first, but do not let that be an excuse for giving up. Keep reviewing your attempts and challenges, and resolve to do a little better each time based on what you learned the previous time.
Never go to a social event hungry, unless you are planning to have a MEAL there. You are just setting yourself up to overeat or eat junky food. The "saving-calories-for-later" strategy does not work well for SO many reasons. Eat before you go if you know there is going to be a lot of finger food. Have a balanced, healthy meal at home and maybe just plan to have a small dessert there. Do not eat anything unless it is on a plate and served all at once (that includes you fixing the plate). Picking at food here and there makes it difficult for you to process how much you have eaten. By the time your stomach feels the stretch and sends the signal to the brain to turn off the appetite, you have already overeaten.
Drink water or Perrier (NOT soda water or flavored water) instead of alcohol, or at least have 1 or 2 glasses of water between alcoholic drinks. This will help counteract the dehydrating effects of alcohol, reduce unnecessary calories/sugar, and prevent you from becoming inebriated! Bring your own healthy dish to share-something you have discovered and thoroughly enjoy. This assures that you will have at least one thing you can eat there that will not sabotage your healthy eating efforts, and makes you look like a thoughtful guest (momma always told me to never be an empty-handed guest). You might even find people asking you for the recipe!