Summarize:

If you are dehydrated, further exertion will only make your body weaker. Stop your activities. This will help promote heat loss from sweat and prevent heat exhaustion or heat stroke. This will prevent any more exertion and help  prevent fainting. If you can, prop your feet up. This may help keep you from fainting. If dehydration is a side effect of heat exposure, remove excess clothing to cool off. You can also use damp towels and spray misters to help cool your body.  Do not use ice water or ice packs. These can cause the blood vessels to constrict and can actually increase heat retention. Use a spray bottle to mist lukewarm water onto the skin. The evaporation will help cool your body.  Place damp cloths on areas of your body with thinner skin, such as the neck and face wrists, collarbone, upper arms and armpits, and inner thighs. If your child is mildly dehydrated due to overexertion, for example from playing vigorous sports, encourage the child to rest in a cool place out of the sun until she has replaced the lost fluids.  Allow your child to drink as much water as she wants to during this period. For older children, sports drinks containing sugar and salts (electrolytes) can be a good rehydration option. Use the steps in Method 3 to rehydrate your body. Drink at least 2 quarts (2 liters) of fluids over two to four hours.  You should try to consume sports drinks that contain electrolytes or rehydration solutions to help restore your electrolyte balance. Mix 1 quart water with ½ teaspoon table salt and 6 teaspoons sugar for an inexpensive at-home rehydration solution.  Avoid salt tablets. They can cause excess salt in the body and can cause severe complications.
Stop activities. Move to a cool area. Lie down. Cool your body. Encourage your child to rest. Rehydrate.