Balance is the key to a lot of advanced mountain biking techniques.
Spend time playing on your bike at low speeds, turn as tight of circles as possible, go off small drops, and practice wheelies things you are usually too tired to do after a long ride. Anticipate the tough climb and shift into a low gear early. Both tires should be in contact with the ground. Be sure to bend at your waist and keep your butt over the top of the seat. On longer climbs, the most common mistake is to take off up the hill too fast, running out of gas halfway up the climb.