Search for KeyRing in your Market place application. Then you simply scan your card barcodes with your phone's camera or enter your card numbers manually. The app does all the rest. Pull up the appropriate card when you're shopping and the clerk just needs to scan your phone's screen.
Key Ring also sends you coupons. The coupons you receive depend on the stores in your card list. Again, you use the coupons directly from your phone.
At this point, not all store scanners can read phone screens. The clerk may need to punch in your card number manually which display on the screen.
