I may be in the minority here, but I use very little cash when I go to Japan. I have about $20 worth of Yen that I got years ago that I carry around with me for an emergency, but I never use it.

I have a Suica card that I recharge ocassionally. If you'll be on the trains a lot, I highly recommend you get one. Plus, you can use the card at most of the stores/kiosks in the train stations.

Other than that I use my credit card everywhere else.