Most crap comes off with a wet rag.
Contacts use Weiman's cooktop cleaner. If it's unavailable in your area, the purest rubbing alcohol you can find is fine. Purer the better. 70% I think is available at most drug stores.
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Nastier things like sticker residue or sharpie, goo gone and denatured alcohol respectively are the answers. Rub gently with a cloth rag. Rubbing too vigorously on the cart exterior is unnecessary and if you rub too hard the friction will begin to melt the relatively soft plastic and ruin the texture, making a bald spot.
Be careful never to get wet things - cleaner, cloth, etc. - near the edge of the front or back labels when you are cleaning. Rub away from sticker edges, not along. If moisture so much as touches the labels, it will permeate them and in some cases raise them and make a blister, in other cases make an indelible oily stain. It is safe to use goo gone and alcohol on MOST label surfaces themselves (they're laminated), again if you mind the edges. Be aware of labels that are torn, have pinholes poked in them, or thin paper labels (akklaim) and treat these areas as gingerly as you should treat the label edges themselves.
It's never necessary to let solutions sit for more than 30 seconds when cleaning, and in the case of denatured alcohol on labels, will eventually eat through the gloss. Gentle pressure. For fine work around label edges, use a q-tip instead of a soft cloth. Apply solutions to the rag/q-tip itself and not to the cart to avoid excess or outright spillage.
Um...what else...oh and finally, if you see a white haze from using your denatured alcohol, lick your finger and wipe it off. You can avoid it by getting the alcohol up as quickly as possible, the hazing is from it evaporating.
I prefer to clean SNES games tho since i dont have to open them to clean them.