Weight Back, Move Forward: As you move down the slope, keep your rear end as far back on the saddle as seems wise without losing control of the bike. If the front end seems wobbly, or skips from side to side, you are probably playing it too safe—leaning too far back. This is more apparent on bikes with front suspensions. They require more give and take. Adjust accordingly by the feel.
Stay Low: Hold your body as close to the bike as possible. Hanging limbs create ungainly movements. Suspension systems may take the jolt out of many rough descents, but your legs and arms are the most effective shock absorbers you have. Keep your elbows bent at an angle slightly greater than 90 degrees and avoid gripping the handlebars too tightly. Relaxing your muscles is the key to maneuverability. How can you tell if you're gripping the handle too tightly? Drum the handlebar with your fingers. If you find it difficult, you're holding on too tightly.
Even if you have suspension, you want your legs relaxed and ready to take bumps. On a hardtail bike this is even more important. To soak up the bumps, get your weight back, grip the seat with your thighs and keep your knees bent and relaxed.
Maintain Your Platform: Your feet should be at 3 and 9 o'clock position. This is called the platform and is the best position for your feet unless you're negotiating tight corners. From this position you are centered on your bike and well balanced. Reacting from this position is a simple process of shifting your weight forward, back, or side to side.
Fluid Descending: Your balancing act isn't the only thing you can do to keep your bike moving down those steep slopes smoothly. The route you choose and your brake control contribute to a fluid descent as well.
Pick a Line: A beginner's mistake is looking at spots you want to avoid rather than focusing on where you want to go. Pick a path and stick to it. Fix your eyes on your chosen path. Scan ahead for future hazards. Make sure your head is roughly parallel to your top tube, and look ahead. Remember that foresight is the precursor to all maneuvering. Knowing hazards ahead of time can help you adjust your balance.
Control Your Speed: You want to blaze down the trail, feel the wind on your face, get that adrenaline rush. Sure. That's part of what cycling is all about. But chances are you are not alone on the trail or road. You could run over an unsuspecting hiker or another bicyclist and that would bring your speed reign to an abrupt halt. This doesn't mean you can't go fast—it just means you need to learn to control your speed.