Being someone who is close to fluent in one language outside of English and conversational in another, Rosetta Stone is a horrible teaching tool. It will show you a picture of a horse or cat and you have to guess the word. Stupid.
You best option outside of immersion is to find textbooks for either high school or college level language course. Read the material that takes you step by step, and do the work. You don't even have to sign up for a class, but it would probably help. Try and get books that come with CD's and they can also help you listen to how the words are pronounced. Amazon has a ton. You just need to know the book titles. If you have trouble finding a good textbook, just e-mail College language departments and ask for book recommendations.
What language are you considering? Depending on the language too, it could be difficult. I have friends who have had Spanish for a few years and are pretty close to fluent. I also have one who has been taking Arabic for 6 years and not even near to being fluent.