Because cosmetics aren't required to have expiration dates, use an indelible marker to note the date you opened them.
Never take chances with anything used close to your eyes, such as mascara or eyeliner; replace each item every three months.
Products applied directly to your skin that have a liquid or cream consistency (foundation, concealer, cream eye shadow and blush, lipstick, moisturizer, eye cream) will last up to a year.
Powders (face powder, blush, eye shadow), which contain fewer oils than their creamy counterparts, can be used 1 to 2 years.
Pay attention to moisture beading, color separating, a change in the original color, or a foul smell. These are all warning signs that the preservatives in the product may have broken down. Be especially vigilant about natural and organic products as they contain nontraditional preservatives, which can affect their shelf life.
Keep your makeup (especially anything you use near your eyes) to yourself. Don't share your products -- and your germs.
Don't add water or saliva (yuck!) to thin out the texture of a product that's begun to clump. Throw it away and buy a replacement.
Avoid dipping your finger (and introducing bacteria) into open containers. Use applicators or disposable cotton swabs instead.
Regularly cleanse brushes, tools, and sponges that can trap oil and bacteria. Wash with mild soap or baby shampoo semiweekly and toss sponges after several uses.
Take note of creams in open pots, which are less stable than those with an enclosed pump, which keeps them fresher longer.
"The FDA requires sunscreens to remain stable and effective on the self for 3 years from the date of manufacture, and a year after the tube has been opened," says Los Angeles dermatologist, Ronald Moy. If your sunscreen has expired, pitch it.