When your dry erase board stops erasing, looks stained and is essentially an unusable eyesore the best thing to do would probably be to purchase a new board with a more durable writing surface.
Since all marker board surfaces are not created equally, you can choose an inexpensive surfaces like Melamine, sometimes referred to as shower board, this is the stuff that the big home improvement stores sell. And people often use it for economical whiteboards; sure the stuff is white, and it may even work for awhile but you will be right back in the same position with a stained marker board in no time.
Of course you could purchase a premium porcelain steel magnetic board which are often warrantied against ghosting for 50 years, but they are the priciest option and it would require you to take down your existing board(s) and install the new ones. This isn't even taking into consideration the expensive shipping options that are required since most of these are so large that they need to ship via freight truck.
So what do you do?
There is a product out there that comes rolled in a tube that has a simple peel and stick backing that has a writing surface that rivals that of the porcelain steel and is as inexpensive as many melamine options. The product is Opti-Rite Lite, it is sold by the linear foot so it can easily be purchased in the proper size to resurface your existing boards. The material itself may be cut with scissors (or a razor blade) and it is very easy to work with. Aside from the fact that Opti-Rite will erase clean for many years, the best part is that it is affordable. You can find Opti-Rite Lite and the family of Opti-Rite products at MyWhiteBoards.com