I don't do well in the heat and will sometimes soak my hands, past my wrists in cold water. You'll be amazed how much that helps.
Also, if you can, inbetween rides, take off your shoes and socks. A lot of heat will escape through your feet.
And if you want, hose off your head before putting on your helmet (if you put on a helmet) the water will drip down your back and cool you off too!
Other than that, take frequent water breaks :)