Early morning or late evening workouts are your coolest choices. Wear lightweight, light-colored synthetic fabrics. Scale back on the intensity of your workout and gradually build back up to what you would normally door indoors or in cooler weather. Drink extra water before your workout. Drink about 8oz of fluid every 20 minutes during your workout.
Water is the best choice.
Don't wait till you get thirsty. Avoid caffeine and alcohol. They promote water loss. Don't let your exercise session come to a screeching halt. Always finish up with a stretch.