I also love this topic!
I took 8 years of French through elementary & high school and college. Lost a LOT of it by the time I actually went to France in 2011 - could still get the gist of written French but all hope was lost for conversation. I came back and signed up for Duolingo and a French meetup group.
I think Duolingo works great, with some limitations. Coming back to a language that I used to know was fairly easy with Duolingo - but the key was taking notes to restore/improve vocabulary while
also attending conversation groups regularly.
I later decided to add Spanish, with no previous study (although some extreme basics thanks to Sesame Street and Love & Rockets comics). This was a little harder at first, because the format of Duolingo doesn't really
teach. But it gave me basics and vocabulary and I researched grammar when I had a question. I also liked going back and forth between Spanish and French lessons - Spanish is a more forgiving language (it seems to mostly care if you can be understood rather than getting caught up on fussy rules), so fun to learn - but I just know and intuitively understand so much more French. Of course, the biggest advance I made in Spanish was when I went to Nicaragua, where English is not widely spoken. I was too chicken to speak the language (and didn't have to because my GF does), but being exposed to mostly Spanish-only conversations helped me tremendously. What Oreo says is true - immersion will get you up to speed quicker than anything else.
I'm still too shy to go to a Spanish conversation group

although I did get to use some of it in Barcelona at the GPF. I like doing the lessons but know I need to start speaking it in order to progress.
My recent endeavor is learning Portuguese, which is a bitch because it looks like Spanish but the accent is really, really hard for me. My brain has no frame of reference to connect the letters I'm seeing with the sounds I'm hearing
