The BBC's World Service (radio) has begun a four part series about polyglots/superlinguists: [bbc.co.uk] - hopefully this is playable from anywhere in the world. There's an interesting bit in this first part about the brain using less space for a language as you gain in fluency rather than needing more of it. (For the next three parts, click on the appropriate link under "more episodes" and return to them after they've been broadcast.)

I think what happens is that it's bulky to store words and grammatical rules the conscious way (where you learn them deliberately and force them into general storage before trying to apply them from there), but when you automate the process and make it subconscious, the data is copied and simplified down to a more efficient, natural form for storage in the right part of the brain for language. The original store of bloated language data can then be overwritten with other things. This backs the idea that you should try to learn languages the natural way without forcing words and rules into your memory, but just let them write themselves into your head the right way all by themselves and in their own good time just through multiple exposure to them. This avoids ever putting them in general storage, and it's the process of putting them there that's the hardest work anyway, so you can save yourself a lot of effort by bypassing that stage altogether.