I don't know about any research on the matter, but I can say 2 things that worked very well for me: 1) take an intensive course first 2) then live in a countr

I don't know about any research on the matter, but I can say 2 things that worked very well for me: 1) take an intensive course first 2) then live in a country where the main language was the one I learned Granted, point 2 isn't easy, but nothing can beat it. The most important point is to use the language on a regular basis. If you don't, you'll forget it quickly. [edited: formatting]