Prepare for heat. You can simply to wear extra layers of clothing once or twice a week on your cool-season runs. Another way to prepare is to exercise indoors on a treadmill or other piece of fitness equipment. With no breezes to cool you, you'll work up a summer-like sweat in no time.
Check the humidity. Before you run, tune in to the Weather Channel or a radio station so you know what you're getting into. You must go easy on humid days, even though it may not feel that hot when you start out.
Run at dawn. Try to run at or before dawn, as this is usually the coolest time of day.
Acclimate. By scheduling at least one run a week for the hottest part of the day, you can help your body acclimate to the heat and humidity. Start with as little as 3 to 5 minutes of running. Increase by 2 minutes each session until you can go for 20 to 25 minutes (or whatever is safe on that day). Aim to run at least 2 1/2 minutes per mile slower than you could race a 10K.
Always stop at the slightest hint of heat problems. Symptoms of heat illness include dizziness, disorientation, or lack of sweating. If you'll be racing in hot conditions, I recommend two heat sessions a week for more complete acclimation. To make up for the low mileage, I suggest you either work in a second run at a cooler time that same day, or simply go longer than you normally would on your next "cool" run.
Start slowly. Or you'll be baking 2 or 3 miles into the run. Thank you, humidity.
Take it easy. Do your long runs a full 2 1/2 minutes per mile slower than you could have raced the distance on that particular day (adjusting for the weather).
Drink lots of water. Get into the habit of drinking about 4 to 8 ounces of water every hour.