i was messing around with my bosu ball today and discovered what seems to be excellent rail balance training
simply stand in front of your bosu ball and jump and spin and land on it (90, 180, 270, 360, 450, etc...). you can also practice switch-ups by simply standing on the ball and doing a 180 or 360 and landing on it again.
basically, treat your bosu ball as tho it were a rail, and you can simulate doing any rail trick you can think of (270 on, pretzel 270 off for example)
while it isn't horribly difficult to do this, landing in a relatively balanced stance can be pretty challenging (especially if your legs are fatigued from previous exercises)
you can really feel it work the small muscles in your groin/core
normally i would post this in the NS Bodybuilding Cult... but its been dead lately.