-Most of the braking power is in the front brake because when you apply 
 the brake, your weight shifts forward and that gives the front wheel more 
 traction.
-To maximize braking power, shift your weight back when braking.  
-In loose terrain, use more back brake than the front.  The front has 
 less traction because it is being "plowed".
-In very steep downhill, move your weight way back, almost sitting right 
 on the back tire.  
-A skidding tire will give you no control.  Therefore, skidding is a 
 very bad practice.
-There are situation where you don't want to brake
    -Never brake when flying.  If you are flying in the air (off a 
     jump, drop off, ruts), do not touch the front brake.  If you land 
     with your front tire stopped, you can expect a huge endo.
    -Don't use the front brake in curves (read turning).
    -When going down hill, don't keep the brakes on.  Instead, 
     feather the brakes.

Others have added:
rdexter@xylan.com (Robert Dexter)
You might also add that the momentum of the spinning wheel can cause the bike
to pitch if the wheel is stopped by the brakes.
Bill Rod [smts!brod@msss.attmail.com]           
I don't agree completely.  I think this will induce skidding.  The front 
brake is the best tool for slowing down under any circumstances.  This 
excludes an induced skid in a turn during a race.  I do agree that a 
little more pressure should be exerted on the rear brake tho'.
Robert Dexter [rdexter@earthlink.net]
My comment applies to stopping the front wheel while in the air
on a jump.  Stopping the rotating wheel while in the air would
cause the bike to pitch forward a little.
Bill Rod's comment about not agreeing asumes I mean braking
while on the ground.  You may want to clear that up.  My comment
*only* applies while in the air.

richard@prl.research.philips.com
-Sometimes consider not braking on a short, technical downhill. If there
is a safe run out, you will have more control letting the bike run, and
going too slow on really gnarly stuff can cause more problems than
floating over it. Save the speed loss for where you have control.
Of course, if the descent is too long or there is no run out, you can do
real damage to yourself this way.
-Environmentally it sucks, yes, but a rear wheel skid can provide rear
wheel steering, setting you up for a better line into the next bit.
Note:- most of my riding is done on tracks that are thoroughly churned
up by horses - the bikes smooth out the mud and _improve_ the condition
much of the time.
(Author's note.  I highly discourage this, but I'm not here to censor, so
I must publish this)