Get The Best Out Of Your Air-con

When first entering a car that has been sitting in the heat for some time, open all windows to allow the superheated air to escape. The air inside a car sitting in the sun all day will always be significantly hotter than the air outside.

Don't think that turning the climate control to maximum cool will bring the temperature down any quicker than simply leaving it on 'automatic'. If it is a manual system, keep the blower speed to below the maximum to get the fastest temperature drop, useful if there are passengers only upfront.
By all means switch to recirculate when driving in heavy traffic, but allow the system to draw in fresh air as soon as the traffic clears.

Most climate-control systems provide a quick-demist control that works to clear the windows when it is particularly cold outside by sending a rush of warm, dehumidified air to get rid of the condensation on the glass. It's usually noisy because the fan switches to the high setting, but use it.
Adjustable vents should be used to send air up not down and towards the rear of the car. The cool air will circulate more effectively throughout the interior this way.

The rest is all common sense. Darker colours get more heated up than lighter ones. Park in the sun and you'll be in trouble.

The Art Of Air Conditioning
The art of keeping the inside of a car cool is not just a question of air conditioning. In-car climate control, as opposed to conventional air conditioning that merely cools and dehumidifies the air, is a sophisticated, complex science. It allows the maintenance of comfortable interior temperatures through automatic manipulation of heating and cooling systems, air-direction vents, fan speed and temperature control. The driver simply 'sets and forgets' the desired temperature on a dashboard control. The system on the new Saab 9-5, for example, actually lowers the relative temperature on the driver's side because the company has found, to maintain alertness, drivers want, and need, a cooler flow of air than others. The company has even developed ventilated front seats that draw hot air from underneath the driver and front-seat passenger to prevent the sweaty, sticky feeling that is an unfortunate side effect of the leather trim. However it is the upper body that provides the most critical measure of how comfortable or uncomfortable you might be. Volvo has found that it is more important to ensure a refreshing waft of cool air reaches faces than other parts of the body. If the face is cool, the perception will be that the rest of the body is comfortable too. Most automatic climate control systems quickly lower the interior temperature on hot days by turning the fan to maximum and recirculating interior air as soon as the engine is started. By recirculating cooled air rather than drawing in hot air from outside, the system works better. When the temperature has reached the desired level, external air can be drawn in again. The system constantly monitors the interior temperature, constantly alters the mix of hot and cold air, and maintains a required setting more accurately than any manually controlled system. The perfect example of just such a system is found on the Mercedes-Benz S-class cars here in India, the schematic of the airflow of which is shown above.