1. Run during the early/ late part of the day. Running before the sun is above the horizon promises you a much cooler run. Not a morning person? Run when the sun goes down leaving just enough daylight to get your run in, but not much heat from the sun to drain you. If your time only allots you to run during midday try to add in a swim after your run for some cool exercise. Runner's World Magazine: Every 5 degree rise in temperature will slow your pace about 15-20 seconds.
2. Stay hydrated. Drink water throughout the day, but don't forget to rehydrate those muscles after running. If running marathon distances it is important to intake some electrolytes during your run to maintain the electrical impulse to your cells to prevent dehydration. Gatorade/ Powerade are a good source, but remember to dilute the sports drinks with water because of the high sugar content.
3. Wear light colored and loose fitting clothing. Light colors help reflect the sun's rays instead of absorbing the rays. Loose fitting clothing meaning not skin tight, but not baggie. The clothes need to be loose enough to allow a breeze, but tight enough your clothes are not hanging off. Don't wear cotton clothing because cotton will soak up your sweat instead of letting it escape. "Wickering" type clothing that beads the sweat away are ideal, or polyester. If crazy, run in the minimum amount of clothing possible. Guys can run in shorts with no shirts, and ladies can run in shorts and a sports bra. This technique allows the body to breath more adequately and eliminate sticky clothes.
4. Start slow. Starting at a relative slower pace will allow your heart rate and blood pressure to incline at a steady rate. Starting your run at a relative faster pace will quickly increase your heart rate and blood pressure causing fatigue more rapidly. The rapid increase also doesn't let your body acclimate to the heat properly for your run. Lack of heat acclimation can result in heat illnesses.
5. Use sunscreen. The sunscreen can not only prevent your body from the ultraviolet rays, but it can also help to keep your body temperature a little cooler.
Stay cool and beat the heat during the last of these summer months.