Originally Posted by San Rensho
Its not an exact science, but in general, more air, less rolling resistance, but less traction going around corners. Less air, more grip around corners.
I keep my front kinda soft, 80-90 and the rear pretty high, 110-120. This way, the front, which tends to slide first, has good grip and the rear, which carries most of the weight, has less rolling resistance.
+1 It's usually a good idea to have the front 5%-10% lower than the rear anyway. This will give you a similar contact patch on both, giving you a neutral slide-out distribution. You can weigh yourself on the bike one tire at a time to figure out what your actual front/rear weight distribution is, and equate that to tire pressure.
Another factor to consider is the road surface. Really bad Texas chipseal is so rough that you might want to go with lower pressure just due to comfort. My Tour de Gruene partner goes much faster on the final 5 mi of the course with his tires running about 20psi lower. His Cannondale just beats the tar out of him otherwise.