I'll start it off with one of my many techniques for biking summers in steamy New Orleans...
I use a 24oz Polar water bottle (polarbottle.com) religiously when the temps crack 95°F sometime in May here. I fill the bottle as full of ice cubes as I can, then add water. No matter how hot it gets, I have one hour of ice cold water (with ice remaining) to drink or squirt on me.
Here is the trick:
Squirt a big mouthful of ice water into your mouth. HOLD IT there as long as you can or until your fillings hurt. Swallow a teaspoon at at time as the water warms up until it's all down. Be sure to lift your tongue a bit so cold water gets under it as there are miles of blood vessels under there. The cooled blood from your mouth will circulate through your head and neck cooling those areas several degrees. You will feel it working.
Every five or ten minutes do that again. DO NOT gulp ice cold water when you are overheated. That could start some nasty cramping.
My extended commute is about 15 miles. It takes about 45 minutes. I always have a little water and ice left at the end.
Add a second bottle for squirting into a helmet vent or down your neck if you are into that. I usually don't do that myself but I have friends who love that.
OK...so what do you Sun Belters (and others) do/wear when the mercury expands to those sweaty numbers?