Back when I was suffering some serious physical problems, even more serious depression, and general burn-out from having worked for so long, my husband did a serious job search so he could work full-time and I could quit and rest. During his search, he was rather surprised at some of the ways in which the job market had changed since he'd last done such a search. Bear in mind that everything I say about work, pay and hours relates to working as a teacher in Tokyo. Wages in other areas are sometimes lower because of the lower cost of living.
One of the points which he realized was that the 250,000 yen for 25 hours standard which was the case about 15-20 years ago had changed to 250,000 yen for 30 hours and often that is 30 "contact" teaching hours and will not include prep time. That means that you may be required to be on the premises for more than 30 hours even though your contract stipulates you work for 30 hours. Generally speaking, teachers make from 1800-2500 yen an hour at conversation schools or in structured teaching environments.
My husband also discovered that most teaching jobs included a small or large number of hours devoted to teaching children. In fact, low pay for teaching kids is pretty much the most frequently listed work in "The Japan Times" and other classified ads in printed publications. If you're in a hurry for a job and female, this is the quickest route to employment though not necessarily the best type of work. Teaching kids can involve accommodating doting or hawk-eyed parents who will eat up your time with meetings or attempt to coerce you into teaching in a different fashion. Additionally, as if often the case in any educational situation, handling kids can be harder than dealing with adults.
The big chain schools (Nova, GEOS, and AEON) seem to be doing far more of their recruiting abroad and less hiring from inside of Japan. Competition has also caused them to increase total working hours without increasing wages, decrease class time from a standard 50-60 minute lesson to 40 minutes, and decrease overall benefits such as paid national holidays.
Most of the chain schools when hiring from abroad also provide housing at a slightly reduced rate for the teachers they hire. The housing supplement can be considered as an additional 10,000-30,000 yen per month in salary depending on the situation. The main thing is that the schools get a deal on the apartments that you wouldn't get. That is, you may not be able to find a place for 50,000-60,000 yen a month on your own so you're paying less rent through your school than you would if you found your own place. However, the school probably owns the building or has a lease which allows them to have the place at a lower rate so they aren't actually augmenting your income and it's possible that they make money on the deal. That doesn't mean you don't benefit from the arrangement though.
In general, working for one of these schools means working harder and dealing sometimes with rigid conditions (such as Nova's system of penalties) on the down-side but also having someone else set you up with a place to live and help you with any of the logistical issues of moving to Japan on the up-side. The wages are more than enough to live on if you don't go crazy clubbing, boozing, or eating out at expensive places. People who don't have connections in Japan or who feel uncomfortable with the uncertainties of travel abroad often find such places a good way of getting a foothold or simply finding a way to spend a year in Japan before going home and doing something else.
One problem with these chain schools for some people is that they tend to be somewhat rigid in their teaching structure. Some of them will monitor lessons and reprimand teachers for not following the method the school follows. If you're new to teaching and uncertain of what to do, these types of places can be helpful in your initial months as you can follow their patterns when you teach. If you're experienced, such places can be tedious and stifling as you can't be creative and even good teachers may find themselves being admonished rather than rewarded for their attempts to be innovative or creative.
Other job possibilities include working in junior high schools or high schools as an assistant teacher. This can be done through the JET program but also via other means. I don't actually have any experience with these types of jobs because neither my husband nor I are willing to teach kids. I've heard a lot of varied reports about such jobs but one common theme is that the teachers are often given little to do or not taken seriously compared to the Japanese teachers at the schools. However, the working conditions (pay for the number of hours, paid vacation time) can be relatively favorable.
There are also random full-time jobs at various schools you can find largely from on-line job searching. These places don't tend to advertise as much in print because of the cost. The main considerations with these sorts of places is that they sometimes operate on a limited budget and may overwork what few teachers they have and sometimes have been known to have teachers sign ambiguous contracts so that they can compel them to do more than should be reasonably expected. Such places have to be considered on a case by case basis. Some are great and some are pretty awful.
There are also a few places that hire teachers to teach in-company lessons on a case-by-case basis. Some of these agencies offer enough work to make a full wage. Others are mainly good as a means to augment your income with spare cash. The good point of these types of jobs is that they tend to pay much better hourly wages (3500-5000 yen an hour in most cases). The bad point is that most places want the same evening hours so it's hard to schedule in enough of these types of jobs to make a full wage and they often require copious amounts of travel, sometimes very far from where you live. If the locations of the companies are sufficiently far distant, the amount of money you make spread across the total time you spend getting there, teaching, and getting back won't seem quite so impressive. Generally speaking, such jobs require core evening hours - from 6:00 pm to 9:00 pm - for the most lucrative pay. You can sometimes get early morning work but the wages tend to be far lower, sometimes no better than working at a conversation school.
The main problem with relying on agencies that contract with teachers for the higher-paying in-company lessons is that there are no guarantees whatsoever, no paid vacation of any kind, and a lot of uncertainty about the numbers of hours you'll get. My former company is one of the few places that contracted for this sort of work in a fashion which would guarantee a certain amount of money each month but even they don't make any long-term promises and your entire day can be eaten up by a fairly gap-filled schedule and travel time.
The bottom line is that you have to make 250,000 yen a month in order to qualify for work visa sponsorship. There are loopholes that allow companies to undercut that in reality. They can sign you up for a contract that says they'll pay you 250,000 yen a month for 30 hours but have you work less than 30 hours a week. Immigration will not check on whether or not you actually make what your contract stipulates and (almost) no one will act as your advocate if you aren't making as much as you believed you might. You have to be very careful to make sure you understand what you're getting into and ask questions to clarify your situation. Also, companies are legally obliged to give full-time employees ten days off the first year and one additional day for each year you renew. If they give you fewer days, they are violating Japanese labor laws.
My husband ended up taking a situation which offered a decent base hourly wage up until 30 hours and a higher wage after 30 hours have been worked. It doesn't offer any guarantees about the number of hours you get per week though. He hasn't had any trouble at all getting enough hours (fairly popular teachers don't) and the working conditions are very flexible. He can pretty much teach what he wants and set his own hours. No one hassles him if he requests time off and he's not pressured to do any extra work of any kind.
The price of this flexibility and lack of hassle is that he gets no paid national holidays, no raises on an annual basis, and sometimes has to kill time between lessons if his schedule doesn't fill up. If the school were to suffer a drop in business, he would also be stuck for getting in enough hours but that seems like a pretty unlikely possibility given the school's well-trafficked location.
In general, you can choose between better hourly wages or a more secure situation which offers lower wages. For newcomers, the secure situation is often a good way to start. Once you know the ropes, the higher wage situations are easier to seek out and find a way to rely on.