I think for dry heat, what you need is plenty of water and electrolytes (I like Endurolytes). But for sticky heat, you need to find other ways to cool down. Rick's suggestion with the ice sock is good. There is a company out there making a similar product, where all you need to is keep it wet and it will keep your neck cool. They're all over the Internet. Poke around a bit.

Several years ago, I found myself bonking terribly on an unusually humid day out there. I was on the edge of bonking most of the day, but one thing really perked me up. It was cold water and Alka Seltzer. I couldn't believe what a difference it made. I've taken a foil of tablets on rides since.