This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
Beyond leaning and braking, suspension helps substantially in
descending. For bicycles without built-in suspension, this is
furnished by the legs. Standing up is not necessary on roads with
fine ripples, just taking the weight off the pelvic bones is adequate.
For rougher roads, enough clearance must be used so the saddle carries
no weight. The reason for this is twofold. Vision will become
blurred if the saddle is not unloaded, and traction will be
compromised if the tires are not bearing with uniform force on the
road while rolling over bumps. Ideally the tires should bear on the
road at constant load. Besides, if the road has whoop-de-doos, the
seated rider will get launched from the saddle and possibly crash.