The key to clear skin is knowing what triggers your pimples. Acne occurs for many reasons, says Dimitria Papadopoulos, a physician at Rockville Dermatology, P.C., in Rockville Centre, New York.

Hormones. Doctors believe that acne is often caused by hormone fluctuations that happen during menstruation, pregnancy, and menopause.

Stress. High levels of stress stimulate your body to produce more hormones, especially androgens, male hormones that have been linked to breakouts. "Any type of stress on the body is going to make acne worse," says Keri, who tells patients to take time out to relax every day and get about eight hours of sleep nightly.

Genetics. Susceptibility to acne is written in your DNA. Acne rosacea, for example, which is thought to be a genetic condition, typically appears around age 30 and affects nearly 14 million Americans, according to the National Rosacea Society. "This is acne associated with skin redness alone," says Tina Alster, director of the Washington Institute of Dermatologic Laser Surgery, in Washington, D.C.

Drugs. Skin reacts to certain medications, including lithium, Dilantin, and many oral or inhaled steroids that are prescribed for the treatment of asthma.

Cosmetics. Heavy creams can clog the pores and cause acne cosmetica.

Hair Products. Styling sprays, serums, and mousses applied too close to the face can cause what's called pomade acne.

Bacteria. Dirt and bacteria congest pores and create acne. However, this is a more common cause of teenage acne than adult acne, because adults tend to be more diligent about cleansing their skin regularly.