How do you learn a second language?

A friend of mine turned me on to, and already I’ve been having fun learning (and refreshing my memory of) some more French.

I’ve tried to dedicate myself to learning a language before, but to be honest, I think that the only way for me to really learn a language would be to immerse myself fully. I’d have to force myself to only speak the language. The linguistic equivalent of tossing myself in the deep end of the pool without water wings.

Have you successfully learned a second language? How did you learn it, and do you have any tips?