Because I study linguistics and teach English, people have often asked me, "What's the best way to learn a foreign language?" Of course, the best way is to move to a country that speaks that language, immerse yourself in it, and struggle until you get by (taking private lessons from a native speaker all the while). Since most of us can't really afford to learn a language that way, I reinterpret the question as, "What's the easiest way to learn a language fast?" In that case, I'd go with audio lessons.
A year or two ago, I found the kind of all-audio course I was looking for in a library. The only problem was that it was for French, and I really wanted to learn Japanese. I listened anyway, and it was great! It was by Michel Thomas and was basically just him teaching two students with little pauses inserted to give you a chance to answer before they do. (Michel suggests pausing the CD as needed.)
Anyway, now that I knew there was something like that on the market, I looked for the Japanese equivalent. (Michel Thomas doesn't make one.) I first got a Learn in Your Car product, which I regretted. It was just one translation after another with no participation required.
The English speaker says a word or phrase (twice), and then the Japanese speaker translates it (twice). That's all! This is by no means useless, but it's not a very good way to go about learning Japanese. I'd recommend this system for reviewing your vocabulary and filling in some gaps. For this to work, you should actually listen to the CDs multiple times. It's the sort of thing you would play while you sleep for subliminal learning.
Conversational Japanese: Learn to Speak and Understand Japanese with Pimsleur Language Programs actually engages you and makes you participate. You're asked questions non-stop, so you can't tune out even if you want to. When you hear a new term, you're not expected to grasp it all at once. It's broken down so you can clearly hear what it is you're supposed to repeat. Plus, you get real conversation at normal speed thrown in. I love it!
I'm not throwing out my LIYC audio, but I'd definitely recommend Pimsleur first. It's a great way to step up your Japanese (or Spanish or French or German...) while you drive or do laundry or whatever.