I always find brake fluid on a bit of fine grit wet and dry gets any marks off.
or you could look here:
How do you clean your Xenarc Touchscreen?
Best way to clean an LCD??
i think the general consensus is a microfibre cloth and possibly something mild and soapy.
Or get some safeclean wipes that should say safe for touchscreens and keep some sachets in the glove box.
(which reminds me i have to pinch some more from work)