http://lifehacker.com/5823069/how-to-quickly-cool-down-your-cars-oven+like-interior
Video Demo (skip to 1:58, it's in Japanese anyway)
Roll down a window on a side of the car. Let's go with the driver's side window. Go to the opposite side door (in this case, passenger side) and open/close it ~5 times. Just do this normally.
SCIENCE: What we're doing here is to help circulate air by drawing the hot air out from the opening/closing door, and causing the cool(er) air to enter the car. So... be careful. Don't accidentally jam your hand against the door. Also, do this when nobody is next to your door so you don't slam it into them.
Video Demo (skip to 1:58, it's in Japanese anyway)