Check the Breeze
If possible, start your run going with the wind and then run back with a headwind. Running into the wind has a cooling effect, and you'll need that in the second half of a run.
Head Out Early or Late
Even in the worst heat wave, it cools off significantly by dawn. Get your run done then, and you'll feel good about it all day.
Can't fit it in? Wait until evening, when the sun's rays aren't as strong—just don't do it so late that it keeps you from getting to sleep.
More: Your Guide to Safe Night Runs
Slow Down
Every 5 degree rise in temperature above 60 degrees can slow your pace by as much as 20 to 30 seconds per mile. So don't fight it—just slow down.
More: Summer Running: Take Precautions in the Heat
Run in Water
Substitute one weekly outdoor walk or run with a pool-running session of the same duration. If you're new to pool running, use a flotation device and simply move your legs as if you were running on land, with a slightly exaggerated forward lean and vigorous arm pump.
Protect Yourself
Take steps to prevent the following hot-weather illnesses (and while you're at it, learn to prevent the most common running injuries, too):
Heat Cramps
Cause: Dehydration leads to an electrolyte imbalance.
Symptoms: Severe abdominal or large-muscle cramps.
Treatment: Restore salt balance with foods or drinks that contain sodium.
Prevention: Don't run hard in the heat till acclimatized, and stay well hydrated with sports drinks.
More: Why Sodium-Potassium Balance is Critical for Better Hydration