Ugh. Try here in Texas in August for the Hotter n' Hell...we have humidity too, so sweat doesn't evaporate. It just sits there in a hot puddle.
I was using water in one bottle, gatorade light or diluted gatorade in another. My semi-pro BF is sponsored by Hammer Nutrition, so I started using the Heed in both water bottles. I felt SO much better that now I carry powdered Heed with me and when I refill I remix the Heed. Hammer suggests Perpetum for long rides, but I just don't like the taste.
And once it starts getting hot, I stay out of the gym. It is too cold in there, and it makes doing anything outside just seem hotter (sort of an unscientific lobster theory....it hurts less if the water heats up with them in it, rather than plunging them headlong into the boil). I find that because I am outside all the time, I can tolerate it better than someone who moves from in to out more often. There may be NO validity to the theory, but it works for me!