I think for me the best way has been to use an audio course in conjunction with some written reference materials...I used these to learn Spanish, and I ended up traveling quite a bit with my skills!
Pimsleur languages - audio courses for learning languages. The amount of levels you can do depends on what language it is. Spanish has 4 full levels, but Twi for instance only has 1.
WordReference dictionary - look up any word in a variety of languages, and get results instantly. Really good for being stumped on something you forgot.
Also you can look for "501 Spanish Verbs" (also available for other languages) at your local bookstore or online, usually costs less than $15. It's a good cheat-sheet for verb conjugations and spellings which can be really tricky for the first few years you speak a new language.
Hope this helps!
