The following is an excerpt from Petfinder.com’s The Adopted Dog Bible
Note: Always be extremely careful when cleaning near your dog’s eyes.
Thoroughly mix equal parts corn starch, milk of magnesia, and hydrogen peroxide to make a creamy paste. The amount you will need depends on the size of your dog, but about a teaspoon each should be enough to get you started.
Apply this mixture carefully to the stained area, taking precautions not to get it in your dog’s eyes, nose, or mouth. Do not let him lick it off.
Keep your dog’s head still, and let the paste stay on his fur for ten to twenty minutes, then wash it out thoroughly. You may want to condition the bleached fur after this process, as it may be brittle. You can repeat this process, as necessary, every couple of days until the stains are removed.
If you cannot easily keep your dog still for the ten to twenty minutes needed for the solution to work, do not risk this method as the solution could get in your dog’s eyes.