Feel free to think about these methods/products that have been used successfully by other users. These are not "official" endorsements, proceed at your own risk. Doing something agressively or incorrectly can cause irreparable damage to your items. This is information volunteered by others in the community to help. No warranties, etc etc etc.
Small piece of Mr. Clean Magic Eraser to gently scrub the contacts on games that appear dull and/or dirty.
99% Isopropyl and qtips, followed by some DeOxit D5 on a qtip to help protect the edge connector. Magic eraser works good if you take the cart apart and pop the PCB out to clean it that way. Just depends on how bad it is.