Practice, practice. Balancing is a skill - it can be learned, but like any other skill, you have to keep it honed.

I always try and practice my balancing skills...I have an old skate deck and a 2 liter bottle full of water I fool around on most every day for a little while. Believe it or not, it seems like it helps my bike balancing skills as well. (e.g. trackstands, manuals)