Don't try the magic eraser! It doesn't do to well with marker, I have a scuffed up part of my desk because of them. Try using an spray can of something (I have tried hairspray and axe body spray) it most of the time fully removes the marker.

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You have to be careful, as some cans of spray have chemicals in them that can melt the plastic (as does nail polish remover, etc). Basically it's the alcohol in the body spray that removes the marker, so Mike Richardson's suggestion of rubbing alcohol is a safer bet as it's pure and shouldn't damage the plastic... YMMV of course!