I have still yet to use this one but I saw it on Seinfeld again the other day. A telemarketers call Jerry he begins to talk to the person for a minute or two then he says he has to go and asks for the reps home phone number so he can call them later at home. The rep states that they do not give out their personal numbers and jerry interrupts and say "ohhh because you don't want anyone calling you at home right? Now you know how I feel!"
I want to use a variation of that on a customer, ask them where they work and then say i'll be in next week for my discount since i'm such a good customer and have been buying from there for soo long. Just to see that look of utter confusion on their face would be priceless.