Isolation from your mother tongue
If at all possible the ideal way to learn a language is by complete immersion as a baby learns its first language. By isolating ones self from your known language and creating a dependence on the new one, it is surprising how quickly one learns.
My first experience with this was on a trip to South America alone with no idea of Spanish. Initially with a phrase book and dictionary to help me string a few sentences together it all fell into place with locals being very eager to help even more. It was only a couple of weeks before I was comfortable with communication and after a couple of months it was assumed by several people that I was a local.
I was 21 at the time, and had no one at all with whom I could speak or listen to in my native English, and after a period of time like this it is natural to start thinking and relating in your new language.
Not all of my languages were learned this way, but this was certainly the best and most efficient way of mastering a new language for me.