Oakleaf and TrekDianna offer good advice:
  1. Always look where you want to go (not were you don't want to end up) -- this is true at any speed!
  2. Relax your arms and shoulders -- no "grip of death" anywhere, but make sure you have fingers wrapped around the handlebar!
  3. Feather your brakes and lower your speed *before* curves, never during the curve.
  4. Experiment in different descents with how you distribute your weight. I generally like lifting my butt off the saddle, and moving it a little back, putting some pressure on the sides of the saddle with my thighs. Pedals horizontal at 3PM and 9PM. Shift up so you can pedal, as pedaling can give you more stability. As I have become more confident, I started lowering my chest and head towards the handlebar, tuck in my knees towards the top tube, elbows tucked in, too. These days I am putting a little more weight on the front wheel, which results in additional speed. Let me tell you it is insanely fun when you dash past people who are pedaling like mad and you are just tucked in. In a curvy descent, you want your hands on the drops for more braking power. My top speed thus far has been just over 45mph, but I live in a relatively flat part of the world...the 45mph was during a race, on a long descent in a highway, pedaling hard most of the time.
  5. If you have a friend that is a good descender, follow his/her line.

GCN has a very nice series of "How-to" videos. See this one on descending switchbacks: http://www.youtube.com/watch?v=EohtLFZcvg4