(This post was really much more interesting and better formulated before I wrote it down, but unfortunately it has been traveling around in my head for over a week and bits and pieces got lost whereas other just became muddled.)
When I was still rather new to language learning, I used to have periods when I was extremely motivated and then periods when I wasn't the slightest bit interested in languages. Perhaps this was due to the rather low level I was at in both of my languages (Russian and Arabic), or just due to lack of experience. I really do believe that you learn how to learn, and that I partly did learn after lots of trial and error.
I would really be interested in hearing from you other folks how this works for you personally. Do you ever - or often - lose interest in your languages? I never lose interest anymore. I think the last time I lost interest was last summer; since then I have had no unproductive periods (and since I always used to get quite depressed during such times I guess it's uniquely a good thing, just so that SOMEONE doesn't start talking about how we all need to rest and so on - we don't).
I was engaged in some other things back in the up-and-down days as well; I used to be a very ferocious knitter, and anyone who has ever been into knitting knows that this is (or can be) an extremely time-consuming activity, especially when you are working on 12 simultaneous projects, something that all hard-core knitters naturally do. I also used to go to the gym quite a lot (yes I know, weight lifting and knitting... I have always liked weird combinations) and that is surely a very tiring activity. I do neither of those these days, both due to inability and lack of opportunity/time/money... the positive effect of this has been a concentration of my interest and energy on foreign languages. This concentration could however just as well have been applied to knitting instead. I can very well imagine a situation where it is completely unimaginable to not knit during any free moment of the day (yes, that's how serious you get) since that was practically what I was like when I was not in my language learning mode, just like it is now completely unimaginable for me to not constantly try to improve my languages. It doesn't really require an effort; not doing anything is more of a task than just naturally keeping myself busy. It is usually not worth pointing out such things to people who have no interest in anything and who don't spend ridiculous amounts of time doing something most people wouldn't even think twice about since you just get weird looks. It is much easier to just go along with their "oh, you are so talented with languages"-excuse or the "wow, your knitting is so even, that surely is the result of TALENT and not hard work!". Talent is a lame excuse, and an extremely annoying one when you don't have any.
Oh oh, I almost forgot. I think (know) that the language learning community (dear Skypers/IRCers, dear Forum folks!) has also helped me a lot. I just can't bear seeing other people hard at work, or speaking about being hard at work at least, while just sitting there myself.
The other day Jack sent me a link to a song that was just incredibly beautiful and ever since I have been listening to Ludovico Einaudi. I will provide you with the very same song that he gave me. I have the feeling it was made for my very own little world of classic literary.