Wear darker, heat-trapping clothing. Roll up the windows in your car and turn on the heater. Don't use the air conditioning. Again, these should be eased into.
In his book, I'm Here to Win, two-time Ironman Champion Chris McCormack discusses the heat training of a fellow athlete who prepared his body for the brutal Kona oven by setting his treadmill up in his laundry room, closing the door, and putting a bunch of wet towels in the dryer on high.
This was not the only training that athlete did, nor should it be the only training you do. Acclimation is but one aspect of heat training.
Aside from sweating buckets alongside a dryer, getting out on the road is also important. General fitness will contribute to how well your body does in the heat.
Just because your race will be 90 degrees Fahrenheit does not mean that all of your training should be done in layers, or in your laundry room. Train in the heat, but recover in a temperature-controlled environment.
If you are out running, either in training or during your event, and you feel tired and need to stop, do not sit and hang your head. The hottest air will be radiating closest to that thick, black tarmac. Putting your head near it will only make you feel worse. Stand up, walk, find some shade and liquid.
Train too much in the heat and your performance will suffer because you will exhaust too quickly to get the cardiovascular benefits. General fitness as well as getting your resting and active heart rate down, will contribute to your body's ability to withstand and adapt to higher temperatures. A higher heart rate means that your body is burning more calories, and that creates heat.
A calorie is a measure of heat energy. Exercise causes you to burn calories. Burning calories creates heat. As you burn calories, your body heats up. In order to counteract that, you sweat. The more calories you need to burn, the more your body needs to cool off by sweating.
The more you sweat, the more you need to drink to replace all those lost fluids. Bigger bodies, even healthy bigger bodies, burn more calories than smaller ones. Bigger bodies create more heat, which means they need to be better at tracking and controlling their water intake.