Wear light, breathable clothing made of a synthetic, moisture-wicking material. These fabrics are designed to draw moisture away from your body, and often have microscopic puckers that allow air to circulate between the fabric and your skin.
Adhere to a work-rest cycle in which you rest for 10 minutes out of every hour. This will allow your heart rate to slow and your internal temperature to cool slightly.
Work out early in the morning when temperatures are cooler, or in the evening after the sun is low in the sky. If you must exercise outdoors while it is sunny, make every effort to spend as much time in the shade as possible.
Use sunscreen, but use it sparingly. Sunscreen stronger than SPF 15 might interfere with your skin's ability to shunt extra heat.
Hydrate often with small sips of water or a sports drink. The commercially available electrolyte replacements are designed to replenish glucose and sodium, which you lose through exercise and sweat. Avoid the belly cramps that come with chugging, and avoid the dehydration that comes with doing nothing.
