How did you really learn a foreign language?
sharpn: I'm learning Spanish at the moment, purely using Michel Thomas's course CDs, and going to Spain for a week every few months. I guess going to Mexico as often as possible would be the US equivalent (I'm in the UK). The CDs are available free in libraries here, so you can rip them for free onto an mp3 player & listen to one per day repeatedly.
I got by well in rural Spain for a week without using English, but I'm far from 100% fluent yet. Other suggestions here like podcasts, radio & TV etc. are also all good. Best of luck!