How to restore dry erase boards that are hard to erase: spray a clean board with wipe dry with paper towels. The fills in the dried pores of the board that hold in marker ink, making it easier to erase. Also removes permanent marker from dry erase boards!
whiteboard cleaning: You can use Air Freshener to clean a dry erase board that just won't erase (because the ink is very dried on) . Spray the board and wipe, it also helps "recondition" old dry erase boards.
Not only do I use this around the house, but I use a "dry" Magic Eraser on my MacBook keyboard. I keep a small piece handy nearby, use, and trim off the dirty part so I have clean sections for the next use.