It pays to do your research
In personal finance it pays to focus on the big wins. For most people, the biggest expenses are accommodation, transport, insurance. In Japan for a lot of people mobile phones are expensive enough to make it into the 'big cost' category.
Most people are aware of the 'big three' in Japan: Softbank, Docomo, and AU.
The big providers have extremely opaque and expensive pricing, and seem to focus on acquiring new customers over taking care of old ones. In some ways, if you don't jump provider every couple of years you are losing out on bonuses, discounts, and incentives.
The big three also lock your phone to their network, although people with newer phones (my iPhone 6 is too old to qualify for this) can get them unlocked at the end of a contract. Having an unlocked phone is really useful when travelling abroad, as you can pick up a cheap SIM and use it for data and the odd phone call.
I am a classic sucker in this market. I have been with Softbank for a long time, tend to keep my phones for 3-4 years, and never look at other options. I find my monthly bills tend to range from 6,000 to 9,000 yen a month, more expensive when I am paying off a handset and cheaper when I have paid it off in full.
I am a light user, with wifi at home and work. I rarely make calls and when I do they are short ones.
Recently (?) in Japan a number of smaller, less well-known companies have entered the market, offering more choice and much cheaper plans.
Of course, they don't advertise as widely or have the market power of the big three, but seem to offer cheaper services. There is an interesting thread in the Forum on this.
I'm currently working on a post about alternative mobile providers. Please tell me about your experiences in the comments below! Any good companies? Any problems with the service?