You all know Woz, but do you know Piotr Wozniak, the Polish memory whizz? Piotr Wozniak created the program Supermemo which is a handy little piece of software that helps people remember things. i.e. new languages. Its not very user friendly, but I'm buying a copy anyway. He has discovered that there's a certain optimal time delay for reviewing things that you want to memorize. Ideally, you should review an item that you want to remember just around the time you're about to forget it. Who knew? If you review an item that you want to remember at that optimal point in time, you'll be more likely to remember it longer.
This explains why the Pimsleur language system works, I think. I can't recommend Pimsleur enough. The Pimsleur language system consists of CD/mp3's that you listen to. As you listen, you repeat back sentences. One new sentence, two sentences that you've heard before. One new sentence, three or four that you've heard before. One step forward, a couple back. All the time, something new mixed in with older items. The review sequences seem quite optimal because provided you repeat things back, you learn.