It might seem counterintuitive, but to prevent acne it's best to wash your face before your sweat session. Wiping the perspiration off your face when working out means you're pushing the day's grime back into your pores. And we all know clogged pores can turn into big red spots. Sweat itself doesn't clog the pores, but makeup sure can. Wipe your face with a cleansing pad, or wash with a mild cleanser before getting your sweat on. Tack your bangs back and off your forehead too to protect the top of your t-zone.
Since the salt in sweat can aggravate acne-prone skin, it's a good idea to wash your face, chest, and back post-workout, too.