A microfiber cloth and a very small amount of water works pretty well. You don't want to slosh liquid on the screen, of course.
At airshows, we have hundreds (thousands?) of people a day come through to poke at things, so you can imagine the glass gets pretty dirty. Part of the morning routine before the show opens is to clean all the displays using these little "CRT cleaner" thingies we bring along... they are kind of like individually packaged "moist towlette" things, soaked in a bit of alcohol (but not too wet).
mcb