If you're American, then you're screwed. If you're in almost every other country, then you are eligible for the Working Holiday Visa. It allows you to stay in Japan up to 6 months with the ability of engaging in paid activities. (jobs)
Unfortunately, if you're American, you have to go through hell to do this, while we watch everyone else sign a piece of paper, then fly to Japan like it's nothing. Kind of weird, but heck, what are you gunna do, you know?
Anyways, here's some more information about holiday working visa:
MOFA: The Working Holiday Programmes in Japan
If you're American, and have some degree, it's still very unlikely that someone will hire you for six months. You're basically out of luck. My advise is to find a friend in Japan, suck it up and buy the plane ticket, then live with you're friend for the summer. But you're not allowed to work.
Lucky me, I'm American. >.<