Check your cosmetic bag every month and discard old makeup.
Many of us have trouble parting with the items we have accumulated, myself included. That said, regularly discarding the old opens the door for experimenting with products and colors.
It's important to note that cosmetics and skincare products have recommended expiration dates.
For example, most foundation, concealer, powder, eye shadow, and cream blush are good for about 1 year. Lipstick, and eyeliner may last for about 2 years, but if you have not used them in the last year, give them a toss.
It's worth noting that mascara, and liquid eyeliner have a much shorter shelf like, and are only good for approximately three months. For this reason, be sure to replace these items with fresh products to avoid eye infection.