1. Never store your products in your bathroom. The heat and moisture in the air when you shower can ruin cosmetics.
2. Don't transfer products into clear jars. They may look pretty, but you're practically begging for antioxidant breakdown.
3. Don't use your fingers when applying products. Use Q-Tips, or, better yet, disposable makeup applicators. If you want something more earth-friendly, use makeup brushes and wash them frequently.
4. Never share any sort of beauty product. We all have different bacterial flora - if we don't feel sick, our immune system has it under control. Unfortunately, if you transfer bacteria to your best friend, chances are, she's not as immune.