While playing with this technique, one thing I found that really helps. Ok... two things really.
First, it makes a difference how fresh the garlic is. This seems to work much
better with fresh garlic bulbs.
Second, if you take a sharp knife and cut away the root before smashing the bulb on the counter, it helps the skin separate more cleanly. The root is what seems to provide the most resistance to the skin coming off cleanly.
Though this probably adds another couple of seconds to the whole process.