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. 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.
You may need to let the bike move a little faster than you would like until you reach firmer ground. 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. Practice leaning your bike into the inside of the turn while moving your body toward the outside of the turn. Your body may resist moving toward the outside of the turn, especially if there is a cliff.