Has anyone found a great way to clean the inside of a Smart Covers? Been a problem since the first iPad, after a few days you get smudges and lines on the screen from closing created by the finger/hand grease that the cover accumulates. Tried plain water but doesn't really work and not keen on using soap/detergent as I'm sure it'd be hard to rinse out. Not to mention it'd probably damage the cover. So, any tips?