(As Dave Ramsey would say :) )
Retailers get starry eyed when you pull out a wad of cash.
Last year I had my car worked on, and the bill came to $740. I asked them (notice a theme? ) if they would let me give them $700 in cash. They didn't even hesitate. For a two minute drive to the bank, I saved $40. This works REALLY well for big purchases. I also did this when I got pest control last year. I asked them if they would give me a discount for paying cash. They knocked 10% off the bill. We've done this with everything from pest control to buying our Christmas tree.
Cash has the other benefit of making you really FEEL when you spend money. It hurts to lay down 7-$100 bills!!! You become more in touch with your money when you actually have to count it out and give it to someone. When I used to use my debit card for everything, a lot of times I wouldn't even hear what the cashier said my total was. I'd just hand over the debit card. A study done by Dunn & Bradstreet found that people paid an average of 12-18% more when they paid with credit cards rather than cash. McDonalds did a focus group study that showed that customers paid an average of 47% more when they paid with a credit card rather than with cash.