I think it's pretty standard fare for low-point FIS racers to do a variety of pretty aggressive balance exercises standing (or jumping) onto a physio ball. Even aging masters racers do this. Check out the illustrated ball jumps near the bottom of page 1 of this thread (admittedly, far less extreme, but still impressive):http://www.nastar.com/forums/viewtopic.php?t=361
IMHO, whole body balancing exercises, especially when you're doing things like catching a medicine ball, also work your core and legs, and seem to mimic the really hard part of skiing (lots of adjustments, like skiing through four inches of cut-up fresh snow on top of ice) which are otherwise hard to replicate in the gym, no matter how many squats and lunges you do.
I think (as a guy who hasn't yet gotten there, but is trying) that the key is to work up to it. Balance is one skill that improves hugely with training, and there are lots of simple, inexpensive balance training tools you can use at home, moving up from 1-footed balancing on a crumpled towel, to using a foam roller, to a bosu, to a bongo board, to a wobble board, to a physio ball.
But padding/soft turf (and maybe starting with balance aids like long poles) seem to be key, especially the higher off the ground you are.