I have a "dry erase" calendar which is actually more like a laminated sheet of paper that I can erase each month to create the new month's calendar.  My kids also have some activity books where they use markers to complete the different lessons/tasks. I have found that a dry cloth sometimes does not remove all the residue and there is a dirty looking film left remaining, no matter how much I try to rub it out. 

So I took the dry erase concept and threw it out the window.  I squirted a little bit of Windex on there, and it cleaned off perfectly.  Now the surfaces are squeaky clean and ready for more writing!