Consult the archives for more information, but start with these potential suggestions from Forrest on NewtonTalk:
Personally, I use a product called Kleen Screen, which is not only for display screens but also exterior parts of PDAs, computers, etc. It is essentially de-ionized water, but works great.
You should never use any harsh detergents or cleaning agents, as this can damage the rubberized finish. (Very) Mild (perhaps diluted) soap and water seems to work pretty well.
Consult the archives for more information, but start with these potential suggestions from Forrest on NewtonTalk: