Pick up the credit card and bend it back and forth slowly. The key word is "slowly." If you bend the card too quickly and too far in opposite directions you may cause an already damaged card to tear. Your goal in this exercise is to take as much of the curve out of the card as possible.
Place the card in the middle of an encyclopedia or another sturdy, heavy book. Close the book and place it on a table. Now, stack nine more encyclopedias on top of the one with the credit card in it. You want to create a weighty compress. Think of how bulky flowers become flat when pressure is applied to their surface. Another idea is to put the credit card in a plastic bag and to stack bricks on top of it. Allow the weight to settle for several days.
Take the credit card out of the books or from beneath the bricks and put it into a credit card slot in your wallet. Such card slots are often form-fitting. Do not carry the wallet in your back pocket. This can lead to further warping.