Flatten your back for aerodynamics, but keep your head up so you can see down the road. Divide your weight between the pedals and seat. Clamp the saddle nose between your thighs to help steer the bike. Grip the bars lightly. Relax your elbows and knees so arms and legs act as shock absorbers.
Some descents are a straight drop, but many have curves or even switchbacks.
The cornering technique you use on the flats works fine on fast downhills. You just have to set up each turn farther in advance, and look farther ahead, because your speed is greater. Click here for a primer on cornering. Your Touch Brake before the corner . Feather brakes lightly to control speed.