Returning to Japan
Although no one seems to have the real numbers, from what I can gather there are about 50,000 tertiary students studying in North America every year. Of this number, about 4,000 are doing an MBA or other Masters course. Outside the USA, there are probably another 30,000 people studying in similar ratios in Europe, Asia, and Oceania. That's quite a lot of people, and assuming that most of them learn English while they're away, it represents about 2.5MM people who've returned to Japan in the last 35-40 years with overseas experience and language skills.
While about half the MBA candidates are sponsored by their companies and therefore are obligated to stay with the company when they get return, the other half and most of the undergrads are faced with finding a job when they get back to Japan.
I find that most young Japanese people living overseas are having such a good time with their new lives and experiences that they don't pay much attention to the inevitable day when they have to return to their own country. Clearly, the prudent thing to do is to prepare well in advance, so that the great adventure doesn't end with a dead-end job in Marunouchi, where English is a mere curiosity and overseas experience a point of suspicion.
I think most people studying overseas are quite interested in continuing to use their English and foreign living skills, so I'm going to focus on preparing for those sorts of jobs.
First and foremost, foreign companies in Japan demand a high level of computing and financial competence. So it goes without saying that your overseas studies curriculum should include some programming (Access, Excel macros, etc.) and basic managerial finance courses. Second, and VERY IMPORTANT, when you select a company to intern at for your OPT work experience, be sure to pick a job that will actually be in line with your job expectations when you return to Tokyo. Often I find people pick OPT internships that seem to be fun or exotic, without much thought to the fact that when they get back to Tokyo this experience may be the only real indication to the recruiter of what they can actually do.
Having aligned your courses with your intended field of employment back in Japan, next you need to start looking for actual positions and understand what companies are looking for. It's never to early to start signing up with job sites such as DaiJob.com. It is probably a good idea, however, to hold off applying for jobs until you are within 3-4 months of ending your college course. Applying before and during your OPT is a very good idea.
Most companies do not want to interview students while they are still overseas. The exception to this is major corporations who are continually after MBAs from ivy-league schools. Competition for post grads from such schools is severe and most people have an easy time selecting which prestigious employer they want to work for. So, since you hope to have at least set up job interviews before you get back to Japan, how do you get companies to respond? One way is to interview on the phone with Tokyo, then offer to go in to the headquarters in the US (or where ever else you happen to be) and do a face-to-face interview there.
Another way to get in for interviews while still overseas is to use a headhunter. Most high-end recruiting firms don't want to talk to people they can't see, but some of the mid-sized and smaller firms will. They are flexible and often the CEO is the head recruiter, meaning that he/she is a good judge of character, even on the phone. Thus, they are willing to interview and make arrangements for when you get back to Japan.
The interviewing strategy that I have seen work quite well is where a person on an OPT internship takes summer holidays and sets up a series of interviews over 1-2 weeks while back in Japan. You can always terminate the internship early if a really good job comes up.