Funny how the same woman who would never consider drinking milk on its expiration date won't think twice about using liquid foundation she opened five years ago.

Nothing lasts forever, and that certainly applies to cosmetics. Once opened, the clock is ticking so use it or lose it, according to Julian Reynolds, owner/founder and lead makeup artist of Julian's Cosmetics + Skincare in Atlanta.

"Lipsticks should be discarded after two years--and lip glosses every year--because you are applying a liquid consistency directly to the skin and there is a potential to develop bacteria in the product," Reynolds says.

In particular, take note of the date when opening a new mascara. Dried and clumpy mascara is a breeding ground for bacteria, due to the wand's close contact with the eyes. And don't pump the wand into the tube, thinking that you're somehow freshening it up. All this does is pump air into the tube, which kills the product faster. You'll notice a very chemical smell in mascara that has been open longer than it should have been.

"We recommend changing your mascara every three months," Reynolds says. "Or if you have had any sort of eye infection or irritation, the mascara should be thrown out and replaced with a new one immediately."

Oxidation and degradation of certain elements in liquid foundation begins as soon as the air hits it. Plus, fingers going back and forth between the product and one's face exchanges bacteria between the two. Add heat and humidity to the mix and the conditions are perfect for promoting yeast and mold. For these reasons, Allure recommends opting for a new bottle after three months of use.

It's not uncommon to amass quite a collection of eye-shadow shades with which to match one's wardrobe. However, it might be best to stick with the handful more apt to be used on a regular basis, Reynolds says.

"If you have not used the eye-shadow in the last five years, it is not the right shadow for you and you will probably never use it again, anyway," he says.

Most women tend to put their faces on in the bathroom but it's really not the ideal place. Keeping all cosmetics in a cool, dry linen closet that is free from heat and humidity is the best bet. And when it comes to application tools, makeup mogul Bobbi Brown suggests washing all makeup brushes every few weeks and throwing out any makeup that is broken, missing a lid, or in a broken container.

But a little common sense goes a long way.

"If it smells funny, looks odd or seems to have changed in consistency or color, it should not be used any longer," Reynolds says.

Quick date check from Bobbi Brown

Throw after 1 year:

Eyeliner

Lip liner

Eyebrow pencil

Throw after 2 years:

Blush

Eye-shadow

Moisturizer

Powder

Concealer

Simple stay-fresh secrets for applying makeup:

Wash and dry your hands thoroughly before putting your fingers into a product.

Avoid reinfection of cold sores or conjunctivitis by tossing make-up used before infection.

Smell your brand-new mascara and note the scent. Old mascara takes on a chemical smell.

Don't cover a pimple and stick your finger back in the makeup.