Yep, what Jones and Ms. Jai said... Be hydrated well before you ride. Or best bet-just ALWAYS stay well hydrated.
But for me I'm like Jones, if I keep the back of my neck cool I'm happy. That is a major heat regulation zone in the body- or so I've been told by my ultra running freak friends- so if you keep your neck cool, your core temp will stay cooler. Of course even that has it's limits. Right now I have to ride in the 104-110 temps of early eveing in Phoenix. It's rough, but like Jai I just try to take it easy. I also take my aerobottle with me filled with icy gatorade to encourage me to injest more salt. I used to only do that on longer rides. But having the bottle right in front of me encourages me to drink.
I drink WAY more gatorade in the summer than I used to. I moved here from nor cal (windsor) two years ago. Heat acclimation was hard for me, but I'm used to it now. I have electrolyte issues so I have to injest alot of salt to stay on an even keel.
Anyway, like Jones said- a wet head and neck will go a long way in cooling you. It's really all you can do besides drink, drink,DRink, DRINk, and DRINK!