I wish I had known about using a Suica Card the first time I visited Tokyo. Not only can Suica get you a discount trip from the airport to town but they also make it much easier to get around on the JR East rail line across Tokyo and make purchases in shops and restaurants.
Suica Cards are the size of a credit card and are tapped on a reader to transfer funds for train tickets or purchases around town. When you pick them up at the airport as part of the foreign tourist package, you'll get a discounted roundtrip to and from the city as well as a balance of funds on the card. When you run low on funds can add more so the card can be used over and over.
Most convenience stores and some restaurants will accept Suica Cards for payments making it very easy to make purchases without having to fumble around with money. This also cuts down on change in your pocket. When riding a train on the JR line you'll no longer have to stand in front of a machine to try to figure out how much your next fare will be. Just walk up to the gate, tap, wait for the pip pip and you're on your way.
Details on using Suica from Haneda
Details on using Suica from Narita