Nice thread here!
I have been living and working in Japan for about four years now and I think it's important to keep in mind where you are aiming at.
I started studying Japanese like seven years ago and at that time I would have never imagined I would be actually moving to Japan, have a nice job and play bands like I am doing now. But always had this kind of a dream to keep me progressing.
Especially when trying to learn the language, many times you would just want to give up.
I have been doing Japanese at university and always took it seriously, but after more than two years I couldn't read most of what was on newspapers and still had no clue of what people where saying on TV. It got slightly better after graduating but as soon as I actually went to Japan I noticed I wasn't anywhere near being fluent. And then after few months I suddenly realized I could finally speak and understand with no problems (o_o).
If you have the money and the chance to have one or two study years in Japan, just go for it. You get to learn more naturally and so much faster than being abroad (especially if you are going by yourself and have no chance to speak anything else than Japanese).
If you are considering about moving to Japan or working there, you can try to get a working holiday first as a tryout. Unfortunately my country has no working holiday regulations with Japan, so I didn't have this chance. I heard many horror stories about Japan before coming, but actually enjoyed it much more than expected. Ok, after a while you get to see all the bad points about being a foreigner in Japan but there are also so many good things too and spending few years overseas is never a bad thing anyway isn't it?
If you think you can get a job because you know Japanese or/and English, good luck I guess everybody's story is different, but most foreigners I know had troubles finding full time jobs unless they were married with a national or had powerful connections. Finding a job took me so much time and I was actually on the verge of giving up.
Even if you are going to study and do not need to work to pay for your school and life, do as many part-times as possible. This way you start getting used to jobs in Japan and some working experience on the field will make you look more reliable at job interviews. You can also meet tons of new people who might be helping you later and of course get more confident with the language. Being able to do something other than knowing the language is also a big plus as the country is already full of gaijin able to translate from/to Japanese or blonde guys who can teach English at language schools.
I made it sound quite hard, but if I could do it I guess everybody has a chance. I am not particularly clever, I don't come from a rich family and had no Japanese acquaintances when I first came. All you need to do is just moving forward without giving up. It's your dream after all isn't it?
Sorry for the long post! If you guys have some questions about Japan or learning Japanese or whatever, feel free to ask