Wannareef taught me to do it with the edge of a credit card. It's thin and flexible enough to get under the foot, but not so sharp as to easily cut it. You just use it to get under the edge of the foot -- once you get a bit up, it'll come off pretty easily. If you're clever, you can cut up the card into a popsicle-stick or guitar-pick shape and make it easier to handle. Just be sure there's no sharp or pointy edges.