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. Use as much rear brake as you like, until it begins to slide. A sliding tire is fairly useless in helping you maintain your speed.
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. You may find that your bike just does not turn. To avoid having your front tire slide out from under you, shift your weight into the outside of the turn and forward. Sometimes you may need to help the bike around by pulling the rear wheel off the ground and hopping it over a few inches.