If your radiator is not working properly, heat can destroy your car. Here's how to cope with heat until you can repair your cooling system.

[edit] Steps

If you have the air-conditioning on, turn it off! The A/C makes the engine work a little harder, which you want to avoid right now. It's actually better to turn the Heater on. The heat will escape from the engine this way.
Flip the climate controls to vent, turn the heater all the way up, and turn the fan all the way up. If this is summer, open all the car windows or you will roast (if you are sitting in traffic on a hot day, this is going to be hard for you). Try pointing your vents out toward the window. Also bring along a spray water bottle with cold water so you can spray your face if you feel too hot. Don't do this in heavy traffic or when it is dangerous for you. Why this works: The heater in most cars works by using extra heat from the engine to heat the incoming air. (This is why it always takes the heat a few minutes to "come up" in the winter.) So turning the heat on full-blast pulls as much heat as possible off the engine and blows it into the cabin of the car.
Turn off your engine, but only if you are sitting in traffic and not moving for more than a minute. Keep a lookout ahead for when the traffic will move and turn your engine on and put it in gear before that point. (This is a huge tip because as soon as your engine turns off the cooling system gets a break.)You want the engine running so the water pump is moving the coolant or the heat will not disapate
Keep it steady in stop and go traffic. It is better to move at a steady slow pace than to go fast, stop, go fast stop, etc. (Generally people will not cut you off in stop and go traffic since everyone is stuck in the same situation.)
Pull over if you think your car will break down in stop and go traffic. Turn off the engine and wait for the traffic to start moving normally. This should only be used in extreme measures, like when the temperature gauge on your car is on the H. Once the traffic starts flowing again, it is better for you to drive faster than slower as more air will come in and cool your engine.
If your engine is really, really hot, you'll probably have to force its cooling somehow. You can do it by dampening a cloth in water - you can use your own shirt if you don't have one - and placing it against the engine block; this way, you will cool the engine, and the temperature change won't be too sudden to make the casings crack. Remember: the cloth must be damp, not dripping; otherwise, it will be pretty much the same as if you were pouring cold water onto the engine.
Take your car to a mechanic ASAP. While the above steps are good when you're in a pinch, they won't help in the long run, especially if your cooling system is shot.

[edit] Tips

If your car does overheat and steam starts to come out from under your hood, you'll have to pull over. You can add coolant (or water if you don't have coolant) to the radiator, but be careful opening it. When you take the cap off, steam will pour out and scald you. Instead, place a towel over the cap and radiator, grip the cap through the towel, twist and pull away quickly. It is better to open the radiator cap while the engine is idling rather than turning it off. Do not turn the engine off to add water. If the cold water you are pouring in comes in contact with the very hot engine it will crack the block and it's then time to replace the engine! It ensures that no steam will pour out when you twist open the radiator cap because hot water is still circulated by the water pump.
If your coolant is leaking somewhere, then you will have to continually replenish it. Pull into locations likely to have a garden hose that they won't mind you using. Churches are usually a good bet.
Always use a proper water/antifreeze mixture in your vehicles' cooling system. Water has a lower boiling point, so your car will overheat and become inoperable more easily with water. In addition, coolant offers other protection to your car against rust and freezing. Water should only be used in emergencies, and after the cooling system problem solved have the system completely drained and refilled with the proper antifreeze water mixture.
In extreme cases, the engine may continue running after you turn the key to off. This is because the engine is so hot that it is auto-igniting even without the electric spark. In this situation put on the handbrake and then put the car into gear - this will cause the engine to stall. If your car is automatic, you'll probably have to force its cooling with the method described above.
If your engine is overheating due to excessive load (such as driving up a long, steep incline or pulling a heavy trailer) it is typically better to pull off to the side of the road, shift the transmission to neutral (or park) and rev the engine slightly (2500 to 3000 rpm). This will allow the cooling system to actively cool the engine while it is under no load and is more effective than shutting the engine off, which only allows heat to dissipate passively. However, if your engine is out of coolant then you should immediately turn the engine off and open the hood of the vehicle to allow the heat to dissipate.
If you are in slow-moving traffic, you can pop your hood. It will stay closed on the safety catch, but open a small gap, allowing greater ventilation (you'll see cops and cab drivers do this in big cities on hot days). DON'T FORGET TO CLOSE YOUR HOOD IF YOU GET TO A FAST-MOVING HIGHWAY! You don't want the hood to suddenly pop up in front of you while you're going 70!
Depending on why your car is overheating you might want to turn the air conditioning on. Some cars have electric fans. The fan is turned on by a computer, based on a reading from a temperature sensor. Sometimes this system will fail and not engage the fan. Some of these cars automatically turn the fan on when the air conditioning is turned on, overiding the failed components.
Engine cooling relies on a temperature differential to exist between the radiator and the surrounding air. Heat is transferred by conduction from the coolant into the radiator core and from the core to the air by radiation. Antifreeze (as well as inhibiting freezing and rusting within the engine) acts as a surface tension reducer, increasing the permissible rate of heat transfer from the coolant to the radiator core, hence correct coolant/ antifreeze percentages are critical. However, if standard glycol-based antifreeze is not providing a sufficient cooling effect, Water WetterTM is an effective substitute.

[edit] Warnings

Under no circumstances should you turn off an overheated vehicle before removing the radiator cap. By having the hood open, the engine running and now opening the radiator cap, you allow a large amount of ambient pressure and steam to escape which would have added to the effects described in the next few sentences. Turning the engine off stops the circulation of the coolant and without the fan runnning allows the coolant to heat up much more quickly thus causing a sudden expansion which can do several things: It can cause a burst of boiling hot antifreeze and steam to come shooting out of the radiator cap. Your radiator/ water pump/ hoses can suffer additional damage from the expansion as well. Contrary to popular belief the expansion/ overfill tank in your car will not function in an emergency such as this.
Do not pour cold water onto the engine or radiator. This can cause casings to crack.
This technique won't work with every car. Sometimes when you turn off an overheating engine to also turn off the fan, and water pump. This is how coolant gets moved around the engine. If this process stops, you keep all the hot water inside the engine.