There isn't much of a secret to language learning because it's all about working with the language as much as possible over a length of time. However I can give you tips on how I learned to speak German fluently (and I mean--I could say anything I wanted or needed to say) within about 6 months. Konjunktiv II? Kein Thema. Polite discourse? Absolut! Telling someone off? That, too.

Don't get me wrong--learning German in 6 months was really tough. I started at an A2 level (the second-lowest level) and busted my way up to C2 (the highest level) and I did it without a whole lot of help outside of class hours. It was really difficult, painful, and even joyful and inspiring. Here's how it happened:
My First German Lessons
First, in the summer of 2002 I took 20 one-on-one lessons with a fantastic teacher at Eloquia, a language school in Frankfurt, Germany. I was in Germany without a whole lot to do during the day and to fill up my time, I took lessons and did homework. This teacher was key--because she prepared me in German grammar construction and corrected me by telling me why something was wrong.
Another benefit of these private lessons was that I didn't accidentally learn other peoples' mistakes! All I had to hear was my voice and my teacher's voice--I was immersed in quality language development.
I then reviewed this material when I returned home and used it to help me write emails to friends in Germany, thus repeating what I already knew. So those 20 lessons actually lasted from the summer of 2002 to the summer of 2003.
Months 1-2
Then in the summer of 2003, when I first moved to Germany, I took two month's worth of lessons at the Goethe-Institut Düsseldorf. I get no money for saying this, and I implore you: paying the tuition for courses at the Goethe-Institut is worth every penny. I took two intensive courses and the instructor doing the assessments recognized how much I could construct in my mind with only a basic knowledge of German from those 20 private lessons. His name is Herr Fluch and he pulled me into his MIttelstufe course.
There was one problem, though...that summer there was a heat wave in Germany and there was no air conditioning, only a few fans. And I actually panicked when Herr Fluch gave us one particular worksheet...because I had never heard of the N-Deklination before. It was hot, the topics were tough, but the saving grace of the class was, indeed, Herr Fluch.