For me:
long, loose, breathable.
-Long sleeves, pants, cover your neck (a wet bandana works wonders there). I like cotton for the breathability and looseness. *I* am much more comfortable with a loose fitting long sleeved shirt flapping around me in the heat, than a tight "wicking" jersey. But different strokes for different folks. What I find helps more than anything (except water) is keeping the sun off and the air moving.

Drink when you're thirsty. Don't be afraid to slow down or just take a short break in the shade.

Don't forget the sunscreen