copying my reply from another thread regarding this:
"bear in mind though that those magic erasers are mildly abrasive, and will "bald" most plastic cartridges, if used aggressively enough -- using an SNES or Genesis cartridge as an example, notice that the plastic isn't completely smooth, but actually has a fine, matte finish -- this can be worn smooth with the erasers, and can sometimes be quite noticeable in the right light. unfortunately, sometimes it's the only thing that works.
i use the aforementioned erasers, Goo Gone, 409, water, cotton swabs, toothbrushes, and clean, lint-free cloths. paper towels/napkins (and even the cotton swabs) can leave material behind in the things you clean, so exercise caution if you decide to use them. also ensure that any electronics you're cleaning (cartridge contacts, systems, etc.) are completely dry and free of debris before applying power."