good yoga poses for balance are the tree pose and T pose(I think that's what it's called). You hold the for as long as you can. To make it harder you close your eyes(really!). They will teach you that centering and sinking your weight down into your feet thing.
Another thing I like to do is sit on an exercise ball like on a horse, then take my feet off the floor(just bring my feet up in back, then put out the arms and balance on it. You'll use your core to stay on, yet also have to relax--the more you stiffen up and force it the wobblier it gets. Do it for like a half hour watching tv--ie your attention is focused elsewhere--and that sense of balance will get into the subconcious. Kinda voodoo heh.
You can make a harder wobble board with a circle of plywood, cut out a small circle in the middle, then jam a softball in the hole.
James, wo ideas--this year I'm doing the lifts with two dumbbells, or I also have a long iron prybar which is about 16#, and is unevenly weighted. Doing stuff like bench press, shoulder press, rows, curls etc with a separate weight in each hand, or a weight that is unsteady because of it's shape(the long bar), is harder than with a single weighted bar. It's harder to lift x amount of weight when it's divided separate between your two hands--one side can't "help" or compensate for the weakness of the other, so you know each side is getting equal work. ALso it's harder to keep the separate weight steady, so you will use more muscle in keeping the weight steady, like this is good for the whole shoulder package(didn't you bust your collarbone?). The wobbling of a long bar in one hand will also work the stabilizing stuff too--and like you can purposely not hold it balanced so it's harder still.
ALso I'm doing sledgehammer workout type moves with that 16# bar--paddling(like a canoe--one side, or a kayak--both sides), shoveling, churning butter, etc, and lots of actual splitting wood