Get a boy/girlfriend who is a native speaker of that language.
I've heard it, read it, even experienced it... there really is no quicker way.
I believe it's true too because it's similar to how toddlers learn to speak. By spending so much time with their mother or primary caretaker... you just talk talk talk and listen listen listen all day long. It's an intensely quick method of absorbing and learning a foreign language.
However, it may not be so easy to find a willing participant ;)
Bonne chance !