Clean cotton cloth (old tshirt or sock) and water. Not dripping, lightly drag (no pressure) over a section of the lcd then using a clean section of cloth procede to the next area of the lcd. Repeat as needed.
Cleaning my astronomy gear has made me a bit obsessive when it comes to cleaning anything resembling optics.
PS. I've read that the ammonia (?) in Windex will yellow the plastic lcd screen over time, so I figure why chance it.