As for my own findings...
One of the things I did last summer: I bought a roll of white contact shelving liner and carefully applied it to the roof of my car. The color change seems to have kept some of the sun from cooking the top of my car as much. This includes the sunroof, which I never used anyways.
I have applied 1 layer of foil sandwiched bubble wrap insulation throughout the entire car. This should help keep some of the sun from transferring heat into the car, but I found that it keeps the heat in even more if you don't have airflow!
Keeping your windows cracked while parked helps, but I'm going one step further. I'm replacing my sunroof with acrylic and built a solar panel. This will connect to a solar charger and a pair of fans. The fans will be in place of my rear speakers and pull air (which will such fresh air either from the windows or through the ventilation system).
I tried doing a pump-powered mister system to emulate a swamp cooler effect in my car (dry climate area). Though misting decently, it just didn't do much. I think I needed a higher PSI pump and maybe to relocate the mister itself to give it more path to absorb into the air.
Another idea I had, but have no clue how to go about it, is if one were to build a tube frame seat and used a mesh cloth to keep it breathing well (see this office chair for example:
http://www.sitcorrect.com/products/MMBM2_4641.JPG). This would probably reduce weight and give you even better cooling than the bead seat covers... though I have no clue how much work and cost this would be.