I have had success with the following: add about 1/2 inch of 3% hydrogen peroxide solution to the pot/skillet with 1 to 2 teaspoons of baking soda (depending on size of the piece). Heat until a simmer is reached, let it simmer for about 10 minutes, then agitate with a scrub brush. I have also read that a drop or two of Dawn will help, but I'm not convinced.