I'm often asked by friends new to the country what's the best way to learn Japanese. The actual answer to this is find a Japanese boyfriend or girlfriend who doesn't speak a word of your native language and go from there. However this is obviously not particularly useful advice for those of a more married persuasion.
At the heart of the issue is that the question is actually the wrong question. The starting point should always be why do I want to learn Japanese? If the reason is that you love the sound, find elegance in kanji and simply enjoy learning then that's a perfect motivator to study as much as possible. However, if the reason is that you're aiming on working in Japanese before leaving after a three year rotation, there's a better way to get a return on your investment.
Japanese as a language breaks down, depending on who you are speaking to, into a number of separate sub-languages. Whether the person is senior or junior, male or female, not only the vocabulary may be different but essentially the language itself. If you're the president in an office, when you learn standard Japanese, it's more than likely your staff would use keigo in response. You may know what you are saying but you haven't a clue what the reply means.
My advice to people coming to work for two to three years has always been the same. Learn to direct taxis and order pizza by telephone but then set realistic objectives. Unless you're an outstanding linguist in the first place you're unlikely to be working in the language just before you're reassigned to a new country anyway. In reality, a little time invested in learning Japan is much more effective than a lot of time invested in learning Japanese.