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I usually don't carry a wallet with me, so if I take a credit or ATM card along, I typically put it into a pocket of my pants. When you keep sitting on your plactic cards, they will start to bend, and eventually break (so you will have to get a new one).
I propose that plastic cards come in two flavors - the usual straight type, and a second model that comes with a natural curve. Of course, using a more elastic material might also work.