not sure if you're BFing, but BM works wonders. i spread a little on DS' heat rash (or even when he scratches himself by accident), and it takes away the inflammation/redness in a matter of hours. i believe BM has vit e in it, so that may be what helps. hth!
Re: Heat rash
Every time Madeline has gotten one we've just cooled her off and it's gone away. I've heard of people using Aquaphor, though.
Vaseline works wonders.
I mix vaseline with cetaphil cream for a creamy lotion for Avery's eczema and dry skin - I bet this would work great for heat rash