I just use a soft washcloth that is slightly damp. Works great. Stay away from most window cleaners, they can have harsh chemicals that will damage or ruin your screen.
I use a micro-fiber cloth and Endust for electronics to clean the smudges. Endust for electronics also helps in keeping a lot of dust from settling on the screen.
a cloth damp with water works best for me. i've "heard" that anything with amonia or alcohol (basically most real cleaners) will damage the screen, or anything plastic in general. you could try contacting the manufacturer also.