IMMERSION IMMERSION IMMERSION.
i went from not speaking a word of German to speaking almost like a native in 11 months. Whenever I go back, people are surprised to hear that I am from America (if they think I have an accent, it's always that the think I am from Netherlands) and I honestly believe it's because I refused to speak English the entire time I was abroad. The more you speak, the more you HAVE to know, and I have found that being shameless in asking questions really helps.
I found that learning sentence structure and a few basic verb conjugations really helped me (to be, to have, to do/make), and then I could just fill in vocabulary from there.
My latin teacher had always touted that an understanding of latin and how languages function would help us immensely in learning a new language, and I agree with that completely. I could always ask someone quickly if a certain verb took the dative or the genitive, which was a solid and quick way of learning grammar. I just needed to note it in a small book that I carried with me and it was in my brain. Also, an understanding of sentence structure (and thereby, the parts of the sentence) given to me in 3rd grade english classes and reinforced in my high school latin classes helped tremendously.