I've done this very thing with Japanese. Picked up learning on my own for a while, then enrolled in evening classes at the local uni. Starting 2nd year of classes in October, with aim of passing JLPT4 next year.
I've used a lot of different methods, and I try to do at least some Japanese everyday. It's been a real struggle, especially with the limited sound system, large number of homophones, and remembering vocabulary items that differ by what seems at first an insignificant single syllable. Take for example: tsukeru, tsukaru, tsuku, tsukau, tsuru, tsureru - each of which is also a homophone! Ugh!
It's now falling into place through increasing familiarity.
Rosetta Stone is ok. It is a big help in the very early stages just to give you a basic familiarity with the sound system and some working vocabulary. It is, though, in my opinion, an extremely inflexible system, and I found that by the time I'd done two units I wanted something that would allow me more flexibility. I haven't used it again since. I wouldn't recommend anyone spend the money it costs.
My favourite resources are websites, podcasts, books and Android apps.
I really like the JapanesePod101 podcasts, and am working my way through them.
I like Tae Kim's guide website, and
this About page on Japanese verbs helped a lot.
I'm a big fan of the Kodansha books, especially All About Particles, A dictionary of Japanese Sentence Patterns, Core Words & Phrases, and Basic Connections.
I loved the old 80s/90s video Let's Learn Japanese Basic (with Yan-san). You should be able to seek it out.
I would completely lost with Wakan (freeware) for my PC and AEDict for my Android phone. Both are useful electronic dictionaries. I also am a huuuuge fan of Obenkyo for Android - the kanji drawing test is awesome! JLPT flashcards for Android is also good.
I'm also making my own flash-cards with kanji on one side and compound words on the other.
Other than that, just immerse yourself as much as possible. Get into the habit of whipping out your phone to practice kanji/vocab whenever you're waiting around for anything. I've exchanged my music for Japanese podcasts and videos on my mp3 player for bus/train journeys and walking around town. Try to read any Japanese text you can. Or even Chinese - try to identify the kanji, and then learn to draw it, and learn some compounds that use it in Japanese.
I agree that classes are the best option, but all of this will really boost your learning. I went into my evening class a bit behind because I'd enrolled in a more advanced class than the amount of prior learning I had. I was ahead of the class after a month or two, though. Not because I'm smarter or more "gifted" at languages, but because I filled my empty time with Japanese as often as I could.