In really hot weather (like we are now having in So Cal) I take before a ride: 2 TUMS (that's right like for a bad stomach but its a great source of calcium), I also like to finish off a Gatorade - either Endurance or Rain and I make sure I have no caffeine before the ride. Also important to take in some food before the ride to help absorb water and keep it in your body instead of sweating it out.

I use only water in my Camelback to which I add 5 - 10 drops of Elete (electrolyte replacement - oderless, colorless and no sugar) and carry a small water bottle for "spritzing" to cool me off. I also carry Enervit which is a salt replacement tablet. If sweating heavily I'll take one or two.

But I think bottom line, when weather is hot, and I'm sweating it's best to just back down on the intensity of my ride to allow my body to adjust. Once
the cramping starts its hard to get rid of them. Pull into a fast food restaurant and order some fries, put your feet up and cool off.