Travellers' cheques: most banks will exchange these, and because a ready supply of cash is essential in Japan, these are recommended.

Credit cards: can be used in big stores, but not accepted as widely as in Western countries.

Cash cards: There are cash machines (ATM) everywhere, but they usually only accept the cards of Japanese banks. Western cash cards will not work, even VISA ones.
The cash machines of foreign banks eg. CITIBANK will accept foreign cards (VISA etc.) , and the machines have instructions in English! (If you find the right button). But there aren't many of them... there's never one around when you need it.
So.... Post Offices are the best bet. Their ATM machines accept foreign cash cards, and have instructions in English.
Bear in mind that the cash machines at Post Offices and international banks are usually inside the building: no access if it's a national holiday. So check and plan carefully around holiday periods.