A polishing cloth, dry except if there are stains on the monitor. If there are stains, I have a 2-wipe kit which I bought from a music store once. They come as a set of 20 disposable wipes, 10 wet and 10 dry - dry follows the wet. One set is enough for a screen upto 30", two are required for larger screens.
In a pinch, a slightly damp polishing cloth will do. I dunno how you can get one here, a guitar store will have them for sure.