The most amusing bit of advice I ever heard on the topic was to get rid of all your credit cards but one, and keep that one for emergency--encased in ice, in your freezer. Then you have to wait several hours for it to thaw before you can use it. The idea is that in that time, the urge to buy would've passed!

Not sure if it would work (definitely wouldn't work unless you wrapped your card in paper and then a plastic bag, so you couldn't see the numbers). But it does make a nice visual image!