I find myself decling the offer of those customer discount/loyalty cards just because I don't like keeping up with all that extra stuff in my wallet. Thanks to friend, Carolyn Bradley, I was introduced to the IPhone app, Keyring.
Amazingly, this will "read" your customer number off of your card and save it to your phone. When you go to the store, just select that store from the keyring list. Not only will it bring up your customer number, it also alerts you to any available coupons. The store clerk gets the coupon information from your phone. No cards or coupons to carry.
I didn't think I had that many cards, but my wallet is reduced by a dozen extra pieces of paper and plastic. I don't have to remember to carry coupons, either. I love this app! There are tons of stores listed and you can sign up for more cards right on your phone. You can registered with Keyring and all your info can be transferred to a new phone. keyringapp.com/download