Combination of methods based on what has worked for me in the past with other languages! I've used Rosetta Stone before, for French & Spanish, and while it's definitely got advantages, I (personally - I also know people who love it!) also found it very time-consuming for very little actual learning, and it's also expensive for what it is.

Basically:

a) I have enough friends who are either native or fluent speakers of Mandarin that once I'm a little more confident with the basics, I will draft them to help me practice conversation skills :)

b) My university offers inexpensive part-time courses to current students.

c) Lots of reading, textbook exercises, watching films, listening to music, translating/reading newspapers, etc. in the language.

d) I'm planning to go to China to teach English in the not-too-distant future, so while I'd like to have basic communication skills down before I go, immersion will definitely help!