I don't usually advocate throwing things away. But there is one category of goods that needs to be tossed on the regular: make-up. We truly do a disservice to our skin when we use the same make-up for too long; the money we save ends up giving us zits, blackheads, even infections.
Here are a few reminders:
Use caution with all things liquid! Every time you glide a wand across your lips or lashes, you're collecting bacteria from your face and putting it back in the tube. You can usually tell when it's time to toss by the smell, which can only be described as “off.” Otherwise, throw away mascara every two to three months, liquid foundation every six months, and lip gloss every year or after you've been sick.
Solid has staying power. Dry solids don't harvest bacteria as easily. As long as your powders are dry, they'll keep for one to two years. The exception is eye shadow because the brush goes from the powder to a mucous membrane, a.k.a. your eyeball, and that can cause infections; toss 'em every three months.
You can achieve optimal performance with clean brushes. All of the above is null if your brushes are dirty, so wash them every two weeks with a mild soap, then air dry.
To offset how many products go into landfills, drop all beauty waste off at a household-hazardous dump in Anaheim, Huntington Beach, Irvine or San Juan Capistrano.