Immersion. For example, if you want to learn spanish, go to Central America and live in a small village with a local family. After about six months, you will be able to carry on a conversation with full understanding. It's kind of the "sink or swim" philosophy. I have been told by many who have done it, that it takes only six months.
ive heard that going to the place that speask that language and picking up the language helps but then again its not like u can just pack up and go to some random country :P so yeah rosetta stone helps