Instead of putting your fingers in your foundation bottle or applying lipstick directly to your lips, scoop or pour out your daily cosmetics onto a stainless steel palette (available at beauty supply stores). Then you are free to use your hands or brushes to apply it. If you never touch your hands to the opening of a liquid or gel product—or your mouth to the edge of a lipstick—you will keep it bacteria-free longer. 
Mascara wand

Don't attempt to lengthen the life of a mascara by "washing" it. Running a wand under water breaks down preservatives and brings germ-loving moisture inside the container. 
Eye pencil

Sharpen your eye and lip pencils each time before use; it will scrape off any newly formed bacteria.
Makeup bag

Don't keep a makeup bag in your car. Heating up in the glove box, preservatives inside cosmetics will degenerate. All of a sudden that skin cream—which is meant to last a year after being opened—is a hygienic hazard. 

Throw out mascara three months after it has been opened—whether it's used up or not.

Keep liquid makeup in the refrigerator, especially if you tend to use a single jar of product for a long period of time and/or live in a warm climate. Make sure to shake liquid products, such as foundation, before using.

If a lipstick hasn't been used in a while, wipe off the top layer with a tissue or paper towel.
Makeup brushes

Clean makeup brushes every few months with a nonalcohol spray brush cleaner and lay on a paper towel to dry.
Makeup counter

Avoid testers at makeup counters. Instead, allow a clerk to apply makeup on you, and make sure he or she uses one-use mascara wands and portions of lipsticks or other creams scooped out with a Q-Tip or small utensil.
Makeup bag

Buy inexpensive makeup bags and discard them after six months of use or after anything has been spilled inside them.
Wash hands

Wash your hands before and after you put on makeup. 

A dermatologist tells all: The top 10 skin myths