How to Cool a Hot Car as Quickly as Possible
To cool a hot car as quickly as possible, start by rolling down the passenger's window and opening the driver's door. Quickly push the door toward the car without actually closing it 8-10 times to force the hot air out of the passenger's window. Then, get in and drive around with the air conditioner on the coldest setting and using fresh air. The car needs to be moving because the condenser for your air conditioner needs air flowing into it. After a few minutes, switch your air conditioner from fresh air to recirculating air to help it cool faster.
Open a window and a door. Roll down the passenger-side window. Then, stand outside your car and open the driver-side door using the door handle. With the window down, you'll be ready to create a low-pressure area that sucks hot air out and cool air in. Fan the door. Rapidly push the door toward the car, stopping just short of actually closing it. Repeat six to eight times, then get in your car. Start the car. Since air conditioning requires air to move over the condenser, running the air conditioning while idling will not cool your car very quickly. Driving around a bit will prepare you to cool your car as quickly as possible. Turn on the air conditioner on while driving. Set the temperature to the coldest setting and the fan speed to its highest setting.  Use the fresh air setting (as opposed to the air recirculation setting), until the inside air temp drops below the outside temp, then switch to recirculate. When you first turn the air conditioner on in a hot car, you'll probably get a blast of hot air. After a few minutes, the temperature will begin to cool. Roll down your windows. Keep the windows down for a minute or two to blast out the initial hot air from the air conditioner and to get the air circulating. Once the air conditioner has cooled the car to a comfortable point, roll the windows up and keep them up. Park in the shade. If possible, always park beneath a tree, in the shadow of a building, or in another cool location. If you're parking in a structure, try to stay off the top level (which is not protected by a roof). Invest in a sun shade. Sun shades are foldable covers for your windshield. To use the sun shade, press it against the inside of your windshield. Open your sun visors to secure the sun shade in place. The best sun shades will be coated with reflective chrome to deflect the sun's rays away from your vehicle. Some sun shades have small suction cups in their corners to allow them to better attach to your windshield. This type can also be used on door windows. Place them in whichever window(s) are facing the sunshine (you may want to park with the front of the car facing the Sun, to use a sun shade there). More than one can be used, but not on the door you need to open to exit the car. Fold the visors down to hold the front sun shade in place better, and to reflect any light that passes over the top. Beware that sun shades can easily blow in the driver's face or out the window once the car is moving, with the windows down. Fold them up and place a heavy object on top, such as shoes, or hold them in your lap, to prevent this. Crack the windows. Cracking the windows a bit can allow air to continue moving through the car, thereby preventing your car from heating much beyond the outside temperature. The more cracked windows the better (one inch / two and a half centimeters each), especially if you can create a cross-draft. Don't crack your windows if you're expecting rain. One risk is that the door locks could be reached by a thief using a coat hanger or other tool, so beware of this. Use a seat cover. If you have dark brown or black seats, your car will absorb more heat than will a car that has light or brightly colored seats. To remedy this, invest in some white or light-colored seat covers. Seat covers are easy to slip over your seats. Like a bed skirt, they remain secure with built-in elastic bands around their perimeters. 
Open a window and a door. Fan the door. Start the car. Turn on the air conditioner on while driving. Roll down your windows. Park in the shade. Invest in a sun shade. Crack the windows. Use a seat cover. 