I agree with everyone who said it's best to practice speaking with others on a regular basis; language is, after all, just another means of communicating with someone
however, anything you can do on the side will help you get the most out of those interactions (engaging in a conversation gets boring really fast if all you know how to do is say that i's raining)
Other than talking, the best ways to study the language depends on what you have available, and what your learning style is
I watch DVDs with foreign language tracks, or at least subtitles when possible, to associate words and phrases with the appropriate context, and listen to audio courses when I'm driving around in my car to get accustomed to the sound of the language
I've also found it useful to think which situations I would be speaking the language(ie. a train station), try to imagine what I would say in English (ie. "Does this train go to the Eiffel Tower?") then learn how to say it in the other language.
That way you can use your time and brain space more efficiently
Two bits of advice I have heard and believe are that a little bit every day is better than cramming in a lot sporadically, and that the key is to learn to "think" in the foreign language, that is, not to associate the foreign words with english words, but learn to associate the foreign words with the things they represent (so it's more like how you learned your first language)
As for Rosetta Stone, I'm still waiting for the consensus on that myself