Our whiteboard has been in use for several years now and it was time to either get it clean or get a new one. See? Before I started to clean it, that bottom part? That's as clean as the whole board would come with the eraser! Terrible.
It should say on the board when you first buy it "Just wipe clean with a soft, dry rag and Rubbing Alcohol" but it doesn't and no one I asked seemed to know a good method to clean their whiteboards either.
Give it a try!