Living in the MS Delta - it's in the 90 - 105 range from May through Sept. I do my best to ride early in the morning. You do eventually acclimate to the heat, but I try not to go outside for long periods when it's 105 and I definitely do not ride when it's that hot. Between the heat and the humidity here, I wouldn't last very long. Most organized rides around here (well - Memphis and Jackson - 2 hrs away) are scheduled for very early AM to avoid heat.

Drink lots just like everyone else has said. Stop at rest stops to refill your bottles when you get the chance. If you use a drink powder - you could carry pre-measured baggies of it with you and then buy a bottles of water at a gas station or convenience store and make it as you go.