As others have said, you can raise a bit off the seat to ease the worst of the bumps, and in time you'll start doing that naturally. I'm often off the seat when riding my hardtail on technical trails, much more so than when riding dual suspension.
I agree with lowering the tire pressure too, particularly if you aren't a really big guy (yet). I run as low as 20lbs front and rear on 2.2" tires, but I'm lightwieght. Wider MTB tires can help too, but they will slow you down a bit.