The only legally "correct" answer is what the vehicle manufacturer recommends
http://www.tiresafety.com/maint/maint_ipressure.asp
However, you will find people with various other opinions, you really have to find what works for you within two limits
1. lower limit - what the car manufacturer recommends on the door plate
2. upper limit - is the max PSI printed on the side of the tire (except as noted in the link above for certain tires)
Here is the best generic formula I have ever been able to find to help determine a more performance oriented tire pressue.
(Vehicle Weight in lb/100) + 2 psi at heavier end + 2 psi all around if suspension and alignment are stock.
Example:
Stock 911, 3,000 lb.
(3000/100) = 30 psi
Add 2 psi all around = 32 psi
Add 2 psi to heavy end = 34 psi at rear
With modified suspension, the result is 30 psi front, 32 psi rear.
"There is no 'golden' tire pressure. The optimal setting depends on the individual driver and his preferences."
http://rotarynews.com/node/view/223