A cycle in the washing machine with regular detergent will brighten dingy shoes. For individual spots, make a paste of baking soda and water (or use a dab of whitening toothpaste) and rub into the stain and then wash.
To brighten leather sneakers, wipe down or spray with a mix of equal parts lemon juice and water and set shoes in the sun for at least six hours. Buff away scuffs with your girl's nail polish remover or a product like Kiwi's Heavy Duty scrub.
If you want to keep your bucks pristine, you'll need a suede brush to erase scuffs. For darker or larger marks, dab hydrogen peroxide on the spot and continue until any unsightly stain is gone.