Exercise early or late. The coolest times of day are best for exercise:
before 10 a.m or after 6 p.m. 

Sweating is your body’s way of keeping cool, and that is a good thing.
But your body loses fluids when you exercise and sweat, so you need
to increase your water intake throughout the day. 

Drink a glass or two before you go out and more when you return.
Bring water with you if you are going to be exercising outdoors for 
more than about 20 minutes. Do not wait until you feel thirsty to drink 
water. 

When you are well-hydrated, your urine will be pale yellow or clear.
Dark urine indicates dehydration. 

Wear white or light-colored clothing to reflect the heat. 

Exercise on cooler surfaces if you can -- grass or light-colored
pavement are cooler than blacktop. 

Wear breathable fabrics, such as cotton. 

Give yourself a chance to acclimate to the hot weather. Reduce your
intensity at first and gradually build up over a period of two weeks. 

Wear sunscreen to keep from burning. Use a brand that is waterproof,
but not oily or greasy, so it will not inhibit sweating. 

Avoid too much alcohol and beverages with caffeine, such as iced tea. 

Do not overdo it. Your body is working harder when you exercise in the
heat, so do not try to perform at the same level that you would if it were
60 degrees Fahrenheit outside. 

If you start to feel unusually fatigued, slow down or take a break in  the
shade. When it is extremely hot, it may help to break your workout
into segments. But do not stop entirely in between; walk or march in
place and keep your body moving. 

Learn the signs of heat illnesses. Stop exercising, get some rest and
fluids and seek medical advice if the symptoms do not subside.