-This skill was intended for track racing, since it is necessary for track 
racers to be able to stay up without putting their feet down.  It is later 
adopted by mountain bikers for doing tricks and stunts.  It is also very 
"cool" to be able to stay up at a stop light:-)
-Find a gentle incline, ride the bike perpendicular to the slope.
-Have your pedals level, with the foot closer to the upslope be the power 
foot.
-Stop the bike gently and turn the wheel toward the upslope.
-Without using the brake, use your balance to try to stay up and not move.  
If you are going to fall toward the bottom of the slope, increase pressure 
on the inside foot.  If you are going to fall toward the upslope, return 
the wheel gently to the center.
-You only need to do minor adjustments to stay up, try to be smooth and 
not jerky.
-Once you can do that, for about half a minute, try to do the same on a 
level surface.