One of the cardinal rules for gaijin coming to Japan is, try your best to make it over here as a university student. College life in Japan is a kind of magical time between childhood and adulthood, mercifully free of the stress that normally comes with studying at university, where you can make lifelong friends and see Japan from a viewpoint you won't ever get to enjoy again. I was too poor to come to Japan when I was in college, so I had to come here as a shakai-jin, a "society person" or full-time worker, which colored my experiences in a different way, but I'd have given anything to be able to visit Japan during school. Many universities offer study abroad programs that allow students to spend a semester or a year living in Japan, so if you or a Japan-focused young person you know would like to come to Japan for a year, start looking into what's available. Remember my theory that every young American should be made to live for a year outside their home country, which would do wonders for the way we view our own country and the rest of the world.
If you are lucky enough to come here during your college years, this is what you get to look forward to: