Listening to lots and lots of flamenco music....juuuust kidding.
Mine is a combination of studied in college, went to a language school abroad, and in-country immersion, though I would say somewhere like 60-70% of what I know was from immersion living here for 4.5 years rather than the other two things. Also as many have mentioned, having an Argentine husband certainly helps, but I feel it's been years since I haven't learned anything new from him, because speaking to the same person over and over doesn't help after a certain point.
So when speaking with new people I still get tripped up all the time, because new people constantly use new expressions I didn't know and it requires more effort to understand them...the other day I heard a colorful new one "dar masa" for to f***. These kinds of things always trip me up the first time I hear them.
Which is why, although I consider myself fluent, I don't think I will EVER stop learning...Maybe I should spend more time with Argentines instead of watching too many Spanish sitcoms and learning phrases that will only get me ridiculed here like "venga, vale, te veo luego!", "qué tío más cansino, joder" or "me está comiendo el coco".
EDIT: another funny one that just comes out sometimes that always gets a good risita: "me tienes en ascuas!" jajaja