You may not be racing in the desert, but there are some things to keep in mind when it comes to exercising in the heat.
Day Time: The time of day is important. Unless you are training for an event that takes place in the daytime heat, avoid exercising from 10 a.m. to 3 p.m. It's the hottest part of day. Generally, the early morning is the best time to workout, especially if it's going to be scorcher that day.
Wear loose, light-colored. The lighter color will help reflect heat, and cotton material will help the evaporation of sweat. You may also want to try specially designed, "hi-tech" running shirts and shorts. They are often made from material meant to keep you cool.
Sunscreen is a must. I use SPF 45 just to be safe. It's important to protect your skin. You can get burned and suffer sun damage to your skin even on cloudy days.
Stay hydrated. Before you go out, drink a glass or two of water. Carry a bottle of water or even a hydration pack such as the CamelBak. Take a drink every 15 minutes, even when you're not thirsty. When you're done with your workout, have a few more glasses of water.
Replenish your electrolyte and salt intake while exercising. I like to use SUCCEED capsules--small, simple packs of sodium and electrolytes that keep my system in check. If you can, choose shaded trails or pathways that keep you out of the sun.
Check the weather forecast before you start your workout. If there's a heat advisory, meaning high ozone and air pollution, you might want to take your workout indoors. These pollutants can damage your lungs.
Listen to your body: Most importantly Stop immediately if you're feeling dizzy, faint or nauseous.