Ideally you want to fill your tires to the pressure where they deform (drop) by 15% of their thickness when loaded. That pressure will vary depending on the load and how it's distributed. This article by Jan Heine in Bicycle Quarterly gives Frank Berto's calculated figures based on the width of the tire and the weight on it. Highly recommended. In other articles (not available online), Jan examines real-world rolling resistance, which is often very different from the figures that tire manufacturers obtain by rolling their tires on smooth steel drums.

On my Surly LHT, with 35mm tires and a moderate load, I run the front tire at 55 psi and the rear at 75 psi.