My favorite tip is "BE AWARE OF WHAT YOU ARE EATING". Pay attention while you eat. Don't mindlessly scarf down rum balls or those little appetizer quiches. Pick it up, look at it, and chew thoughtfully while noticing how it tastes. This means you can't really eat while carrying on a conversation, unless the conversation is about the food. Eating mindlessly while talking at gathering is the source of endless amounts of calories. It also slows down your eating and you will eat less because you give your body time to become satisfied.
Another one is "IF IT IS NOT WONDERFUL - DON'T EAT IT". Even if it was made by your sweet grandmother, even if you just paid a lot of money for it. If you don't think 'wow! I really like this' after the first couple of bites, don't finish it. You may have to eat a bit for politeness' sake (if your grandmother is involved) but you are not obligated to finish it. I hearby give you permission not to clean your plate if you don't love the food. :)
Eat "SMALLER PORTION SIZES" assuming you have control over your portions. Take half the amount you think you will eat. If you want more you can go get more after you are done with what you have
The last is "DON'T CLEAN YOUR PLATE", especially if you have no control over the portion size. Eat half, give it a few minutes to settle and ask yourself if your really want/need more. Then eat half the remaining portion & repeat.