I do not all mean to be flippant, but the answer is...(drum roll)...PRACTICE!

Seriously, find a slightly inclined surface where you can apply a bit of pressure to the pedals without moving forward while slightly turning your wheel in that direction. Also initially, you should use neither a too steep nor a too slightly inclined surface--find something in between. Then...PRACTICE, PRACTICE, PRACTICE! I've actually taught someone with good balance initially to trackstand (somewhat wobbly) in less than a half-hour. (BTW, at first I might not try this while clipped into the pedals.)

Good luck. Check back and let us know how you're doing!