1) Take it easy, build in extra time for breaks and don't use the ride for "training"-just get there and get back.
2) Hydration, drink a lot in advance of your ride, don't forget about electrolytes-too much water can flush these important minerals from your body (it's called hypokalemia/hyponatrema, I believe a DC bike cop died a couple of years ago of this while on a training ride in excessive heat), .
3) Getting the load off your back might be helpful, however, that's a personal decision-by pack sits low enought that it's not a huge trap for heat.
4) Cool down-I'll get right in a cool shower when I get home today (along with my recovery drink or Gatorade) then air dry on the bed before I change.
5) If you're not comfortable with the idea of riding in this heat, it's not a bad time to resort to public transportaion-there's no shame in it, especially in such extreme conditions. In fact, I've advised my wife to stay off the bike this week-she's out of shape and I'm concerned for her safety.