Tomato sauce and other acidic foods leave stains on plastic that even the dishwasher won't remove. Set those plastic pieces out in the sun to naturally bleach the stains away. (Rub lemon juice on especially stubborn spots first. Baking soda also works well in bleaching color out of plastic, and it helps get rid of strong odors.) A great way to prevent stains is to spray with PAM first and then put the food in.