Rather than fumbling for notes and coins and dealing with a train-ticket machine each time you want to ride a train or subway, instead buy and use a pre-paid plastic travel card. They’re called “IC cards”.
There are now two types of such cards.. the plain commuter version, and the tourist version. The plain one is $5 and you can reclaim unused credit from it when you leave Japan, but the tourist version is free and only valid for 30 days and credit is not refundable. You buy them at any train station from a machine and must load credit onto them when purchasing. You can choose to add more credit anytime you need it. You then use them to get into and out of train stations (tapping them on the top of automatic gates as you enter and leave train stations) until the credit is exhausted then you load more cash onto them.
They don’t save you any money but they sure save you a lot of time and hassle.
Tokyo has two brands of cards (Suica and Pasmo) but they work identically and are usable in all the same places. Other cities tend to only have one brand of card. And the cards can be used inter-changeably across all cities in Japan (so, for example, the card you buy in Osaka can be used in Tokyo, and vice-versa).
To enter a station’s automatic gates, you only need enough cash loaded on the card to get you to the next closest station (say, about $2). If you try to get out at a more distant station, you will be blocked from leaving until you top-up your card’s value – which you can do at the “Fare Adjustment Machine” which is inside the station somewhere. So always carry some cash (say a 1000yen $10 note) in case you underestimated the cost of your trip and have to top-up your card’s value.
The cards are not just for train travel. They now can be used to pay on most city buses, on many lockers, at convenience stores, on many vending machines, and at many small snack bars.
For more information, just Google “suica JR-East”.
Japan Tourist Tips 