first off are you sure its heat rash?? what are you doing to cause the rash like owrking outside in the heat , getting sweaty then drying out with the sweat on your skin?
I get rash somtimes on my back and chest from wearing a vest all day , I'll get sweatty then cool off then sweatty again and it breaks me out bad , I try to wash down realy well then after I'm dry I get the wife to powder my back with Gold Bond medicated powder and that helps alot.
Also for you guys that work in the heat alot the Gold Bond with Menthol is great to powder up your balls and butt with before you go to work , keeps your stuff feeling cool and helps keep dry also.