I personally, take a wash rag, drench it in HOT water, wring it out till it's barely moist, then wipe the screen down, usually makes them nice and clean and preety again. :3
Agreed with MoM above but it also seems vinegar and distilled water can be used as well.
To prepare the solution for cleaning LCD monitors, you will need deionized or distilled water and vinegar or iso-propyl alcohol - Don't use tap water or mineral water as they could leave white marks on the screen because of the dissolved salts.