Great point about forgetting the material. If you really want to learn something you need to review it more than once, and you need to encounter it in more than one environment / context (unless you're a genius, maybe). It's science, yo.
I learned that from math literature. To really solidify a topic you need to express it in as many ways and "different angles" as possible. It's related to the more ways you've seen a problem solved the better your understanding.