-Practice on level ground with no obstacles.
-Sit down and have weight slightly forward.
-Shift to a low gear.  
-Push down hard on the pedal and shift your body weight back and pull 
 hard on the handlebar.
-Spread your knees out and try to keep your weight back.  Keep pedaling.
-If you feel you are going to far back, touch the back brake and you will 
 fall back down.

Some added:

toadhall@echo-on.net (SLEW)
The trick to performing the front wheel wheelie is in finding the balance
point where you are able to ride on one wheel, and you have pulled back too
far and will land on your butt.  One way to find this threshold point is to
literally pull back TOO far...and don't worry you won't land on your butt.
Just make sure your feet are out of all manner of locking devices (toe
clips, clipless pedals, et al), and when you are ready to do the wheelie,
pull back as far as you can until you literally fall out of the bicycle...be
prepared to put your feet down so you don't hurt yourself, and instead just
run with bike still holding onto the handlebars.  It might help to lower the
seat a bit so the bike can slip through the legs.  One of the hardest things
to do when starting out doing the wheelie is overcoming the fear that you
will pull too far back.  The best way to overcome this is to pull too far
back then CATCH YOURSELF, then you will have a better idea of where that
threshold point is where you can balance and ride.