The three S’s-sweat, showers, steam rooms-are like kryptonite for hair. “Small water particles can make tresses swell, creating pouf,” says hairstylist Jet Rhys, owner of the two eponymous salons in San Diego. Give your hair a pre-workout spray from roots to ends with a dry shampoo like Oscar Blandi Pronto Dry Shampoo Spray ($11, sephora.com), to absorb water. To prevent fluff, swap your usual conditioner for one that’s rich in proteins, such as Aveeno Nourish+Moisturize Conditioner ($7.99, drug-stores), which help fill in porous areas and reduce flyways. If you have relaxed hair, toil frizzes by brushing strands flat around your head, then covering with a silk scarf before you sweat. Afterward, blast roots for a few minutes with a blow-dryer on cool.