I don't know that there is any way to improve your balance on a horse .... without actually riding the horse!!! And I agree that bareback riding is awesome for working on balance issues.
I suppose you could do something like yoga to work on balance and flexibility in general. And being on a regular exercise program is going to make your body stronger, overall, and more able to hold a balanced position. But again, you kind of need the horse to work specifically on balance while riding.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.