Swig often. To replace the moisture that you lose as you perspire, be sure to drink. As you lose water to dehydration, your body temperature rises, so replacing fluids is essential to keeping cool. Avoid beverages that contain alcohol, caffeine, or lots of sugar, which are dehydrating. "Also opt for hydrating foods," says Deena Kastor, a marathon runner and an Olympic bronze medalist. "Try a smoothie for lunch, and add more fruits and vegetables to all your meals. Watermelon has the greatest water content of any food out there."
Eat light. There's a reason we reach for salads in the summer. They're easier to digest than, say, a fatty hamburger, which leaves you feeling sluggish in the high heat. Instead, go for fruits and vegetables, which are watery and help keep you hydrated (and cooler), says Robert Kenefick, a physiologist at the U.S. Army Research Institute of Environmental Medicine, in Natick, Massachusetts, which studies the effects of extreme climates on soldiers' bodies.