In my experience, tire pressure is often an important pre-ride check that many just skip. However, your tires are not only there to give you traction, but they also are an important part of your suspension system. So, tire pressure effects both how your bike will handle and how long (or short) those inner tubes will last.
Make sure to check & set your tire pressure cold. Changes in air temperature, altitude and riding have an impact on tire pressure. So, for example, say you set your tire pressure in the morning when it's cool, you ride hard for some hours and take a break in the afternoon when outside temperatures have soared. Before you take off again, check your tire pressure, as most likely it will have risen and over inflated tires for the conditions reduce traction and can make the bike feel harsh. In the opposite direction, under inflated tires can increase traction to a point, but can also reduce high-speed stability and increase the chances of pinch flats and wheel damage.
In terms of setting tire pressure, in my experience, if you ask 10 different riders about what pressure is best, you'll get 10 different answers. However, in my own personal experience, what I've been able to gather from others on TT and articles I've read, here's a general guideline on tire pressures (psi):
Hardpack - 11.5 front, 11 rear (no rock or bigger square edged bumps)
Intermediate - 12 front, 13 rear
Sand & Mud - 12 front, 10 rear (if rocky, bump up rear 1.5-1.75)
Obviously trails are not going to have the consistency of terrain that is possible with the controlled environment of an MX track, so zeroing in on a single setting for single riding area is pretty tough. That said, a good baseline setting is 13 front and 13.5-14 rear. However, you may need to go has much as 18 rear for high speed, rocky desert riding.
Generally speaking, typical off-road tire pressures are going to range from 11-15 psi. Of course, other factors include the tire's construction, tire size (sidewall height), inner tube thickness and rider weight. That said, if you're not sure, ask others with lots of experience for the particular area you are riding or experiment yourself, making small changes, logging them, riding and seeing how those changes effect traction and handling.
Regardless of what you do, don't overlook this important step, it really does affect your bike's performance, so setting your tire pressure for the conditions is about the biggest performance bang for buck you'll find.