Heat rash is a fairly vague name for a few conditions that have one thing in common – you guessed it, getting a rash when it is hot.
Most commonly it is caused by a blockage of sweat glands. This means that sweat leaks out into the skin and because sweat is a very salty substance this causes the irritation, usually resulting in either small, clear blister-like pimples, or a more diffused red pimply rash. It seems that there are some bacteria that can contribute to this, and studies have shown that sufferers of this type of heat rash have three times as many bacteria living on the skin as usual. A cool shower when you get it can help clear the pores, and using a skin brush or body scrub as well as an anti-bacterial body wash with tea tree for instance can help prevent it. I would not go crazy with the skin brushing when you have the rash though – your skin will be sore enough. Also try an aloe gel topically to help cool the skin and reduce inflammation.
There are also some people who have an allergic reaction to the heat, much the same as we do to nettle stings for instance. This can cause more of a hives like rash with larger raised welts, and can also cause feelings of fatigue that can accompany many strong allergies. Again, an aloe vera gel topically can really help, as can quercitin as a supplement to reduce the allergic tendency.
Finally, there is a theory that many so-called heat rashes are actually allergic reactions to sunscreen ingredients, and certainly conventional sunscreens contain a lot of ingredients that you could have an allergy to. Try a natural sunscreen instead to see if this helps.