How do I stop rashes from sweat developing on my leg at night?

It's heat rash, I always have it every year when the warm weather arrives. My quilt is thick but I can't sleep without it because I feel really uncomfortable. The rash is at the back of my legs at the same height that my knee is. I've been to the doctor and he gave me some cream but I want to know how to prevent it. It's really bad at the moment, parts of my skin has come off and it bleeds.