When I'm on my roadie, I find that being able to roll/push the bike backwards a bit (followed by sight forward pressure) simulates how I trackstand on my fixed gear. The trick is the rock the bike under you while keeping your torso relatively still (with minor balance corrections, obviously). Once I learned to do that, it became MUCH easier.

Also, when you are first learning, it sometimes helps to keep the top tube against your front leg (this helped me until my balance got better).