With some really oppressive heat and humidity over us here in the mid west (I know those of you down south get it worse) I have noticed a number of vehicles broken down on the side of the road. This kind of heat is just as hard on your vehicle as the bitter cold (sorry for those of you down south you can't even compete on that one). So how do you keep your vehicle running well in this kind of weather.
If you haven't changed it recently you might want to consider doing so. A good flush and fill will go along way to ensuring that your car will stay cool and continue running. Also if you haven't checked your coolant level recently do so and fill as necessary as you will want the maximum amount of cooling capacity. If you have a vehicle that has been modified to produce more power you factory cooling system my not be able to keep up. A little trick to ensure that these vehicles stay cool is instead of using the standard 50/50 mix of antifreeze and water is to run almost 100% pure water with a product like Water Wetter to lubricate the pump (also it decreases the surface tension in the water allowing for more cooling). Granted this mix doesn't have as high of a boil over protection, but will keep your engine running much cooler since 100% water has a much better thermal transfer than a 50/50 mix of water and antifreeze.
If yours isn't working as well as it use to you may just need a new cabin air filter. Changing these is similar in difficulty to changing the engine air filter but a little more expensive with the filter costing $10 to $15. If this still doesn't help you may be low in refrigerant in the AC system. Unless you know what you are doing I suggest you take your vehicle to a professional as it is pretty easy to over fill the system and then wreck seals thus making your poorly working AC into a non working AC
Tires should always be properly inflated, especially in hot weather. Too little air and your tire will over heat and you will be suffering a blow out and have to change a tire in this crappy weather. Too much air and your tire may just burst and you will be stuck changing a tire in this crappy weather.
Heat kills batteries so if you have a battery that is old and questionable you might want to change it out before you get stuck. Also keep your battery properly charged will go along way to preventing the stresses that can prematurely kill a battery.
In addition to providing lubrication to your engine oil also helps cool it as it flows. In extreme heat you should ensure that you have the proper amount of oil so as to maximize the cooling capacity. Additionally using a synthetic oil will decrease the internal friction thus helping your engine stay cool.
If you have the opportunity to park in the shade or in a covered ramp I suggest doing so. Not only will this keep the interior of your car from becoming an oven, but it will help protect interior trim and the pain. Heat is what causes the plastic in things like dashes to dry out and crack. Also by keeping your vehicle out of the sun it helps protect the paint from UV damage thus helping the paint prevent your car from rust. If you can't park in a shaded area I highly suggest getting one of those windshield shades and cracking your side windows to keep the interior heat down.