-Keep your pedals level (3 and 9 o'clock)
-Get your weight back.  The steeper it is, the more you move your 
 weight.  It is not uncommon to see someone riding down a hill almost 
 sitting on their back tire.
-Think positive.  I had the problem of thinking I'm always out of 
 control, but in reality, I'm not even riding close to my limits.
-Shift to the middle/large chain rings.  This will increase tension on 
 the chain and you won't have so much chain slap.
-Brake with mostly your rear brake.  You will still need to use your 
 front, but the back is used more often and harder.  
-Braking the wheel until it almost stops spinning is good.  Skidding is bad.
-Steer with your shoulders perpendicular to the path you want to move.
-Sometimes if you can't ride down some section because it's too bumpy, you
 might want to add some speed.

Others have also said:
Dave Blake [dblake@eureka.wbme.jhu.edu]
NO NO NO ! ! !.  Your front brake always has more power than your back.
Endoing is not a real problem if you learn to modulate your front brake
with the terrain.  Less brake over obstacles, and more brake when the
braking surface is smooth and clean.

lrtredwa@rdyne.rockwell.com
Always be looking for your line.  Identify those spots in the descent where
it flattens out a little, allowing you to brake harder and "get it back".
This gives you the ability to "let it go" in the more difficult parts for
control because your line will take you to the part where you can "get it
back." 

Brian Adams [adams@cs.unr.edu]
-On long descents, consider temporarily lowering the saddle, making it easier
 to get your butt low (or behind the seat) on steep sections.