It's been hot AND humid here in Missouri for the last few weeks. The problem with high humidity (80%+) is if you get your jersey wet to cool you off, it never dries out

A trick I learned at hockey camp last year was diluting Gatorade. Fill your bottle halfway with Gatorade (or whatever other sports drink you want), then fill the rest with cold water and shake. Supposedly, this helps keep you from getting a really high electrolyte burst and then crashing when it wears out. Something about keeping the levels steady. All I know is I don't get loopy as quickly, nor do I get an upset stomach like I do on full-strength Gatorade. I've found that Gatorade powder works better for this, and doesn't taste as nasty - just use half or 3/4 of the recommended scoops instead.

Also, to beat the humidity, I would take an extra jersey or wicking t-shirt and change halfway through the ride (on a long ride, anyway). Less to evaporate out of the shirt means more that it will evaporate off your skin and keep you cooler.