I use 3M's Screen Cleaner:
http://www.amazon.com/3M-Cleaner-Monitors-.../dp/B00004Z5N4/You can probably find it cheaper too. I can't recall how much I paid last time since it lasts a good long time, but in addition to the little bottle of spray, it comes with the anti-static non-scratching wipes.
Or else just search for screen cleaner at any electronics or computer store. But I like my 3M stuff. And even I, who always smudges windows, don't smudge with this stuff. :)