The secret to remembering material long-term

There is this Polish guy who has been working on repetition spacing for a while. He seems less into usability, though, and I think there is an opportunity here for someone who wants to make this kind of stuff simpler.

http://supermemo.com

(There are some interesting articles on sleep too, if you are not intimidated by walls of text.)

In my experience, repetition spacing seems to work for memorizing stuff. I'm less sure about learning skills- operative knowledge-, as opposed to passive information.