Teaching English in Japan has many attractions to many people. You can live in a developed economy, learn Japanese (a very good idea), learn martial arts, live in the home of manga, and learn about Japanese arts/music. But the stagnant economy and low birthrate have dampened the ELT market.
The average starting salary has stayed about the same for years, and even dropped in some cases. However, there are still plenty of ESL jobs in Japan advertised online. As always, the best jobs are found in the country, after you have some experience and connections there.
It's always hard to say what a typical salary is as it depends so much on experience (in Japan), certification, luck, connections, and also how good your Japanese language ability is. Entry level salaries are easier to gauge, and typically range from 220,000 - 270,000¥ a month. The JET program pays 280,000¥ in the first year.
When you have spent some time teaching English in Japan you can find private students. Then 400,000 - 600,000¥ a month is possible. Some people report earning more than this. But to do so you need to have experience, contacts and luck. And, unless you're very lucky you will be working all hours, with little time for anything else.
You need a degree for teaching English in Japan. TEFL certification is not so important, although it can help in schools with a western management, and of course it may help you personally with your teaching.
Teaching experience is most valued if it's from Japan. If you have experience outside of Japan, it won't be valued as highly. However, it will make you a better and more confident teacher. This, in itself, can help when applying for jobs there.
In Japan the visa belongs to the teacher, and when you leave your school your visa stays with you, not with the school - as it is in Korea - this makes life easier for teachers, but can mean that some schools prefer to hire teachers who already have visas, rather than go through the application process themselves.
Most people teaching English in Japan work for eikaiwa (English conversation schools). These are private language schools and come in all sorts and sizes, from big chain schools, which can be a good way to begin teaching in Japan, to small independent language schools. These smaller schools offer similar pay, but can have a more relaxed atmosphere.
The JET program, which is run by the Japanese government, provides a good opportunity teaching English in Japan. It is a popular entry level job, and pays more than the average eikaiwa. You need to apply from abroad, have a degree or be qualified to teach in public schools, and is mainly targeted at people in their 20's or 30's. You don't need to have teaching experience or qualifications. Most teachers will be working around 35 hours a week, from morning to late afternoon.
Private elementary and secondary schools, and universities are another alternative. These are often sought after jobs, with better pay than the private language schools. Positions in universities are often filled by word of mouth, and teachers are usually on contracts. An MA in tesol + published work, as well as teaching experience are required. A PhD is helpful, but not essential yet.
It is legal to teach private lessons in Japan - but this will not qualify you for a working visa. Therefore, for most people, this is just a way to supplement their income.
The best jobs teaching English in Japan are usually only open to teachers with experience and contacts in the country. However, many jobs are advertised online, and some of these can provide a good entry into the Japanese job market. Where possible avoid using agents when looking for work. This is general advice and true for searching for esl jobs in any country. For more detailed information on teaching English in Japan take a look at the website Teach English in Japan
Jobs are available all year round, but the academic year begins in April, so applying a few months before this would be a good time. This would allow time for your visa to be processed. Looking for jobs on the spot, after arriving in Japan, is possible, but risky. Make sure you have a good amount of savings if you decide to do this.
Think about where you want to live in Japan. Tokyo has a lot going on, but it's very expensive, and because of the expense, you will probably have a long commute. 2 hour commutes (each way) are not unusual, and if you have split-shifts (also quite likely) you could spend a lot of your life commuting – with the associated drop in standard of living. Choosing a smaller city would help improve things here. And not that rush hour in Tokyo can extend from around 4:30pm to very late in the evening.
Again, I recommend starting to learn Japanese before you arrive. This will not guarantee you a job, but it might swing an interview in your favor, and will certainly improve your prospects once you arrive.