We teach our students to be loose on the bike. Mountain biking is a lot about anticipation and adaptation. one minute your weight is back, and the next you need to push your body forward for the rear wheel to clear an obstacle. practice rolling around and bring your weight front, back side, side, and i mean ALL the way in each direction.

when you go through sand, you may want to gear down, and keep your weight a little bit back... however, if you are climbing, you are probably going to be "shuffling" your weight front and back in order to maintain traction.

hope some of thie helps

Han