What I've found to work great for me is to ride curbs. bunnyhop up, ride off, bunnyhop up, ride off, bunnyhop up-try to stay on the curb for as long as I can etc. etc.
It doesn't sound like much, but when you do it for a few hours it works great.
Plus, after a while, you get really good at riding in a straight line, so that helps a ton with the balance skills. I can track-stand at the red-lights damn near completely still.
I do it for the present, for something to do on my rides, and I do it for the future for when (if) I get to be 80, so I won't be falling over as a senior citizen and busting my hip or something equally lame.