I make myself use cash as much as possible. Overspending with plastic is way too easy. Something about feeling the dollar bills leaving your hands makes it so much more real and harder to part with then just sliding a plastic card through the machine. You have to stand there and count out the money and see all of those bills leaving your possession and into that cash register. It sobers up any shopping spree when I think about how many 20 dollar bills I'm going to have to fork over for what I have in my cart. I do those cart clean outs right there in the middle of the aisle. "Do I REALLY need this?" and I start putting everything back.
Maybe I'm just frugal.