How to clean off keyboard marks on screen?
If you are very lucky it is just dirt and can be wiped off but usually it is permanent and there isn't much way to avoid them. I have seen it happen even when a soft cloth is used to cover the keys when the lid is closed. In fact that may even make it worse as the cloth adds thickness to the sandwich and may make the contact between the high points of the screen and the keys firmer. The screen surface of an anti-glare display just isn't very hard - glossy glass is harder but can still be marked.

Using a quality micro fibre cloth with a 70% water/30% isopropyl alcohol solution is a good way to clean the marks off if they are not permanent. I have been using the cleaning solution provided by my optician for my glasses for years with no ill effects, and I love the iKlear blue microfibre cloths (for both glasses and displays). They call it "terry" as opposed to their "chamois" cloth.