Experiment with sliding your body forward when seated, and keeping it backward when standing. What you DON'T want to do is allow your weight to be too far forward. In general, the steeper the descent, the farther back your weight should be. It is not unusual to keep your body in back of the seat on very steep descents. Use enough front brake to help keep your speed under control, but not so much that your bike might hit an obstacle and come to a stop or that your front tire might begin to slide.
Loose rocks can pose special problems, making it difficult to brake without sliding.
Many of you will find that you can turn sharply in one direction but not the other. This is probably the side that you tend to get on and off your bike on. To avoid having your front tire slide out from under you, shift your weight forward. On steep downhill turns you must compromise between keeping your weight back to maximize rear braking, and keeping your weight forward to keep the front wheel from drifting. On VERY steep downhill turns you may need to turn the front wheel sharply, shift your weight onto it, and ride it around the corner.