According to N.Y.C. dermatologist Francesca Fusco, it's how you prep your skin before working out—not how you clean it afterward—that prevents breakouts most effectively. "Wipe your face with a disposable cleansing cloth before you begin. Exercising with makeup on can clog pores and obstructs the normal production of oil, sebum, and perspiration." To mattify without causing zits, dust your T-zone with a tinted mineral powder like one from Colorescience (colorescience.com).