Splash water over your face and head at any opportunity. Wet hair takes a few nice cool miles to dry. Drink plenty. I find biking sandals cooler than shoes when it's really hot. And I can go wade in any open water I find as well.

I've heard good things about the cool-you-down bandanas (they have something inside that stays wet or cool), but I've not tried them.

Wear a helmet with lots of big holes. Sadly, the more money you pay, the bigger the holes and the cooler the ride. (Others may not mind hot heads, I do)