The method I explain in this post can be applied only if you leave in a country where people speak the language you want to learn.
In a recently published book Author John Kay argues that goals are often best achieved indirectly. Similarly, I think that we can often learn a foreign language more effectively while we are doing something apparently unrelated to study the language itself. The trick is, of course, to use the foreign language to carry out activities that we find anyway interesting and stimulating.
For example, in a previous post, I described how I use improvisation theatre to learn German. This works well for me because I really enjoy practicing improvisation theatre. Consequently, I could spend hours doing improvisation theatre because I find it funny.
Similarly, as I am very interested in photography, I am considering attending a photography course in German. Of course, in this case, I would dedicate a few hours to learn photography-related terms such as focus, aperture, and light before attending the course in order to maximize my understanding.
Recent research findings suggest that teaching non-language courses in a foreign language improves language learning.
I think it is important to accept, when using this indirect method to learn a language, that one may not understand everything and that some information would be lost by doing a course or any other activity in a foreign language but that it is ok because it is part of the game.
I do not think that this kind of strategy to learn a language should totally replace the desk study of a language. It is still very useful, for example, to learn new words by using traditional methods such as flashcards. But by engaging in activity that you are interested in your target language you will dramatically increase the exposure to that language without feeling the burden of an intensive language course.