You probably are already doing most of these but here's what I did while marathon training in Florida:
1. Drink, drink, drink the night before
2. Brought water in a Fuel Belt and took a gel w/electrolytes every 45 minutes (your timing may vary). For some people, taking gels with gatorade can cramp your stomach and cause ugly things to happen. During the training runs, I figured out my fueling and hydrating needs and I never had any problems racing in the heat. Also, when it's hot, some people take Hammer Nutrition e caps to replenish electrolytes, but I never could tell a difference.
3. Wore a Nike Dri Fit visor.
4. Run in as little clothes as possible. I'd rather have the air circulating on my back than a Dri Fit top plastered on
5. Before you run, you could always plant a half frozen bottle of water on your route.
6. Know you'll be at least 10% slower due to heat and humidity.
Sorry I don't have any better tips!