One thing about living in an Urban Area without a car is that I don't go to Supermarkets often. I usually do my food shopping at any of a series of small mom and pop markets. There is one that has good fruits and vegatables, another that has good meats, another that is cheaper on the staples, etc, etc. In fact the majority of time when I am in a supermarket I am away from home. This has lead to me cultivating a talent to a level that few Americans have acheived. That talent is the ability to talk a checkout clerk in a strange supermarket into running through the Bonus or Club card so that I can get the discount. Now I know what you are thinking, they will do that for everyone. Not so. I can think of a few chains where initally they say they can't. Then I pull out depending on the age and gender of the clerk one of the following chestnuts:

Young female Clerk: "Listen I am actually visiting from New York City for a college reunion any chance we can get some leeway"

Female clerk that clearly has more than one kid, "Sorry I am visiting my Mother and she has the card, but I just ran out to grab a few things for her" (Doesn't work when your basket is 4 cases of beer, four bags of chips and a Nerf Football)

Male Clerk: "Yeah when my girlfriend dumped me she took the card"

My biggest advice though is win before it ever becomes an issue, pick the aisle as far away from the floor manager with the youngest clerk with a smile on her face. One quick smile and all of a sudder instead of a 12 pack of hot dog buns being $1.19 you are getting them for 87 cents.