To clean the display, use a non-alcohol based cleaner like a LCD cleaner. Be sure to remove all dust, static, and sweat. For the pen nib, use a soft fabric to clean it gently.
iKlear Spray Bottle
3M Screen Cleaner (Alcohol-free version)
Note: Do not spray too much or wipe too hard. It's recommended that you read the specific cleaning product instructions before using it.