In Atlanta - part of the Sweat Belt - we either deal with high temps and high humidity or wait 'til October to ride. I agree with
beng1 - the simplest and most effective solution is to just ride early in the morning. The past two weeks saw temps near or over 100 with high humidity. But at 6am, the temps were in the mid- to upper 60s. My club rides at 6am during the week. On weekends, we start at 7 or 7:30 depending on the expected noon temp. Even down here, it's rarely above the upper 80s by noon.