I've lived in Texas my whole life. We get the 100+ temps every summer.

Dealing with the heat is three parts hydration and one part acclimation/conditioning.

I used to think that whole acclimation thing was an old wive's tale. However, studies have now shown that the more time you spend in the heat, the better your body gets at conserving fluids.
Also, for every molecule of glycogen your muscle cells take in, they take in a certain number of water molecules, (sorry, I don't know the ratio). So, the more you work your muscles, the more fuel they need and the more water they take in, thus helping you to stay hydrated and cool. I've seen this in my own body. If I take a few days off from commuting, I'm usually feeling dehydrated on my first ride in to work. But I'm feeling fine, by the time I ride home that evening.

So...

Hydrate
Hydrate
Hydrate
and
Acclimate