This tip I picked up a few years ago with a merchant and collector of vinyl records. He used the petrol to clean vinyl skins, remove adhesive remains, price tags, etc. Therefore, I decided to test it in my boxes two newly acquired Master System II.
Quite liked the result. Applied the petrol with a cotton, not to scratch anything. It removes any fine dirt accumulated over the years: dust, fly poop, tail remains adesivos, etc. All this without damaging the ink on the cardboard. Another advantage is that it dries quickly and leaves no residue, not at the risk of staining the packaging where it has some porosity, As with products containing oil. The boxes have gained new life after cleaning.
In more extreme cases, a magic sponge pode ser utilizada com certo cuidado, but it takes the ink gloss in some kinds of box, then I believe that cotton is more recommended in most cases. For boxes that are already very opaque due to cleaning with abrasive materials, a tip is to use a acrylic spray varnish, as or Acrilfix. It is available with a glossy finish, semi-gloss and matte. Using the bright version, cria-se uma película transparente brilhante sobre a caixa, which restores shine and helps protect it. The Acrilfix tem proteção UV e fungicida, which should also help protect.