Well, for starters, to give yourself any chance of gaining an English Teacher's job in Japan, you will need to physically be here. With such a huge supply of native English teachers, why would a school want to employ you? You have to give them a good reason to bend over backwards to get your work visa.
Some good reasons would be:
- Being already in Japan and ready to start asap.
- Hold a CELTA or equivalent certificate.
- Show them you have prior teaching experience.
- Willing to teach in rural areas where other teachers are not interested in going to.
- An attractive, positive and mature personality.
Just have a backup plan if you are unable to gain employment. I do have friends here in Japan that are from Europe and are teaching English. However, they have been sponsored via other means such as a "translators visa" etc.