Try to anticipate the surface changes and adjust accordingly. This is the beginning of true mountain bike enlightenment. Your position relative to the bike has to be free and changing. To accomplish this, keep your hands loose on the grips, wrists and forearms relaxed, elbows and knees bent, and back straight at a comfortable angle. Practice changing stances by raising and lowering your body as well as sliding your weight fore and aft, shifting weight to the front and rear wheels, respectively.
Relaxation is the key to success.
A tense rider will have to overcome antagonistic muscle activity before a sufficient reaction can occur. A relaxed rider has the advantage of being able to tense the right muscles at the right time, resulting in a much more efficient and explosive position change. Learn and push the limits of the stances you take, and you will soon overcome areas that you could never before conquer. Think of getting your front wheel through a rough section, and the rear will follow with relative ease. Shift forward for more front wheel control through the straights.