In California I did plenty of centuries in 90-100 degree weather. There's no good substitute for just getting used to the heat, but other than that the regular drink a lot of water and eat enough.
I did 119 miles last weekend on 4 24oz water bottles and 4 clif bars, but it wasn't too hot. One thing I try to avoid is too much electrolytes - water them down at least 50%, especially if you're drinking a lot. If you're having any trouble during the ride, don't stress over stopping in the shade for awhile or dowsing yourself with water, either can make a world of difference and you'll ride better for miles.