Are you sure it's from sweating and not an allergy to something?
What are you doing that you're sweating so much? Maybe take a week and take it easy - like if you're an athlete or fitness guy, take a week off, if you can, and see if the rash gets better?
When the rash started, did you start doing anything new? Start using a new soap, lotion, laundry detergent, toilet paper, start wearing a new kind of underwear...? Did any product change their formula, like "new and improved"? Think of everything that comes into contact with the area that has the rash, and see if you can think of something you might be allergic/sensative to.
Are you wearing cotton underwear? Try all white, all cotton underwear and see if that helps.
If you are able to, have you tried taking benedryl to see if that helps?