Moving our Tree?

We've got a rather interesting addition growing on our kitchen ledge. No fancy setup or anything, but with some time and a lot of luck, maybe a tree.

The little baby tree was originally crammed between the slats on our deck, in a tiny pile of dirt. I think hubby just thought he was being amusing when he brought it in and put it in water in a little shot glass. Months later, it's still hanging on to life! Go figure.

Last week Maggie and I decided it was time to upgrade our mini tree to a pot with some dirt. It's been almost a week now since I put the little bugger in a giant (compared to our sapling, anyway) pot with some potting soil. So far, so good! It seems happy, and I'm resisting the urge to water frequently, but I'm not sure if something that was living solely in water for months can be overwatered....

Any advice on raising a tree from a twig to something life-sized?