1. rub non-gel (just plain white variety) toothpaste over the affected area, then gently wash it off
2. rub mayonnaise over the affected area and let it sit overnight, he next day wipe it off with a clean cloth
You can also try pastes made of bi-carb and water, or of salt and vegetable oil. I didn't give these a shot, but anything's worth a try if you can just whip the ingredients out of your pantry.
The mayonnaise was the most effective in this house.