The private conversation school (eikaiwa) industry is your best bet for securing a job teaching English in Japan. The government JET program still hires foreign teachers, but the amount of jobs available has dropped sharply and competition has increased. The monthly teaching salary in Japan is about $2,000 at the entry level. Because Japan is an expensive place to live, that $2,000 leaves little wiggle room for savings or a dispensable income. You can expect to put away only about $400 a month, compared to $1,000+ in China and South Korea (unless you are extremely disciplined about dining/entertainment and find shared accommodation). Unlike teaching jobs in other Asian countries, free airfare to Japan from your home country is not a given – many schools will expect you to pay your own way, so make sure to clarify this point with prospective employers.
Japanese regulations make it a bit easier and much less legally perilous than other countries for prospective teachers to arrive in the country first and secure work later. If you’re considering this options, you should be prepared with funds to tide you over for 2-3 months if needed and a well-researched plan of attack for when you arrive.