Water, water, water. Start drinking hours before your ride and drink constantly during the ride. If you wait until you're thirsty it's too late and you'll have a hard time catching up. If you notice that you stopped sweating, it's too late. The body is like a leaky radiator, as long as you keep putting water in it it stays cool.
Oh, that "dry heat" I've heard mentioned, that's a misnomer. I lived in the southern states for 30 years and know what 100 degrees and 98% humidity is like. I moved to Arizona and experienced 117 degrees ambient and 132 degrees 2 feet off the pavement. I can tell you matter-of-fact, 117 is freakin' hot man. Moved back to the Southern states last year and dang, a "hot and humid" day here ain't so bad after all.
In Arizona, when it was summer, I'd go through two jumbo water bottles on a 20 mile ride and arrive back at home drained and out of gas and water.