For one, go slowly and rest often. Also, try hiking in the early morning as this is usually the coolest part of the day.
Summer provides a great opportunity to explore trails at higher elevations where it's naturally cooler. Keep in mind, however, that the summer season often brings afternoon thunderstorms to many mountain locales. Never ascend above tree line when there's lightning in the vicinity. And if you're already above tree line when a thunderstorm approaches, try to descend as soon as possible.
Wear moisture-wicking clothing made of polypropylene or polyester to carry sweat and moisture away from your body. Moisture-wicking material keeps you dryer, cooler, and more comfortable than a sweat-soaked cotton shirt. It's also a good idea to wear light colored clothing because it tends to reflect heat away from your body.
Wearing a hat—a baseball hat, or, preferably, a wide-brimmed hat—will help protect your face and neck from the sun. Don't forget sunscreen either. Sun-burned skin will make you feel hotter.
Know the Signs
Finally, you should be aware of heat related health issues
on the trail. As part of your first aid training you should know the signs for heat exhaustion, heat stroke
and even hyponatremia; and know what to do if someone in your party exhibits any of these signs.
Hang your sweaty clothes at one of these campsites