Part of it's just practice and figuring out where your pedal needs to be. If you're going that slowly, all you need to do is bring the pedal around to where you can unclip (top of the pedal stroke, usually), unclip, and then brake and dismount. The other thing is that the more you ride and the better your balance gets, the more stable you'll be able to keep the bike at low speeds.

The other thing is to figure out where to shift--it's almost like you need to shift before you need to, if that makes sense. Shifting under load's not a great idea, but it can be done.