Choose one that comes in a mirrored compact for convenience. And if you're concerned about keeping your updo up, pack a few bobby pins for a late night lift. There's wisdom in this advice even if you don't have a cold. Use one for cleanup after the finger foods so typical of parties. And for freshening your breath, a few mints can work magic.
Car troubles can happen at any time.
Also, you may decide to leave the party at different times, so count on carrying money for a cab along with an extra house key. A little change for a telephone call or vending machines is wise, too. And, yes, cell phones are compact enough to slip into even the tiniest of evening bags.