i am always somewhat confused about what pressures to run for trail riding, but what seems to "feel" best to me (i weight about 170lbs with gear) is 28 psi front, and 36 psi rear on maxxis high roller 2.35 single ply's, give or take a very few psi. any more on the front and it feels sketchy, less and i feel the rim getting tagged on bigger rocks. on the rear, less than 36 and it feels like the bike won't roll fast enough (and rim gets tagged), more and i can feel the back skipping over too much stuff, braking is bad, and too much boot-boot. i tend to try to run as much pressure as i can get away with though, because i have more fun riding when the bike rolls faster - this is for general trail riding. but for bike parks i run 2.5 maxxis 2-ply dh tires with 27 psi front, and 35 rear, exactly. for dirt jumps i run 40 psi front, and 50 psi rear for railing berms at high g's - although this can vary more since you don't have to deal with terrain on flow trails.
why more in back than front? because i DO look at my tires while i ride! one of the things i do when testing a different tire pressure is do a "tire sag" check - i sit on the bike and look at the tire to see how much it bulges at a certain psi. i can't explain here how to judge this, but keep in mind that a tire works best on a certain size of contact patch that will vary with psi, tread design, rider weight, casing design, tire size, rider weight distribution, riding style, dynamic forces while riding, and other factors. as part of my riding style, i try to keep the front end light as often as i can by keeping my weight on the pedals (not on my hands), and when riding faster and more aggressive (which i do as often as possible) the weight distribution shifts more rearward. add to this that the weight distribution on my bike is already 40/60 front-to-rear. so, the weight distribution calls for more pressure in the rear and less in the front to keep things balanced-out.