1. Keep makeup in a cool, dry place.
The consistency and wearability of a foundation can be altered if you keep your makeup in the car on a hot (or cold) day. Liquid and cream products are especially sensitive to extreme temperatures. Leaving your makeup exposed to air for extended periods of time will also alter it.
2. Use it or lose it!
Some people save their expensive makeup only for special occasions. Well, how often are those special occasions? Chances are, if you're holding on to that Chanel foundation after three years, it's not going to work as great as it used to. You also run the risk of breaking out from old makeup.
3. Be very careful about cross-contamination!
I suggest purchasing a foundation that comes with a pump that allows your to pump it onto your hand or onto a pallet. Cross-contamination can re-introduce bacteria from any blemishes or acne to your face next time you use that foundation.