DO NOT USE GLASS CLEANER, most of them contain ammonia. I personally use a micro-fiber cloth, and if the mark wont come off with that I use 91 % isopropyl alcohol. I have read online that LCD's are cleaned with alcohol before they leave the factory. I have e-mailed Viewsonic (the maker of my LCD) and they said that occassional use of alcohol is OK, but ammonia based cleaners are not. Ammonia will strip the coating off.