It's gonna sound gay, but when I have to work in 100+ temperatures, I take a damp hand towel and position it on my head so it falls down my back. Put a headband on over it, then wear a baseball cap (bill forward, thank you very much).
Yes, I look like a shiek looking for my camel, but it serves to let me work for several hours in that heat without problem. I stop every hour or so and dampen the towel again.
It's cheap. It's easy. And they may laugh at you or beat the livin snot outta you for suggesting it, but it works.