http://www.alljapaneseallthetime.com is by far the best resource I've ever seen for learning Japanese by yourself. The method is basically:
  1. Use Remembering the Kanji to learn a couple thousand kanji in a couple months
  2. Use Remembering the Kana to learn hiragana and katakana in a couple days
  3. Immerse yourself in an environment in which you are always listening to the sounds of the Japanese language, so that you build up your comprehension
  4. Read lots of manga and watch lots of anime. Whenever you don't understand a sentence, look it up and add it to a virtual deck of flash cards in an SRS program like Menmosyne. Your vocabulary will increase rapidly, and by the time you reach about 10,000 sentences, you'll be as fluent as a native speaker.
Read through the whole site (he recently added a table of contents, which helps greatly). If you're prepared to put in the time, nothing's stopping you from aquiring perfect Japanese in a year.