I've had good luck with using dish soap since it is geared towards loosening up oily and baked-on stuff. About once a month I wash my light blue polyurethane cover in the sink like dishes. I put about an inch of water with enough soap to get alot of suds. Then I scrub the front and back with a microfiber cloth.
I like this method because it removes ALL the built up oils on the soft side and removes any stains on the cover. I let it sit overnight and it looks completely brand new the next morning.
I started doing this because the soft part would get so grimy from my wife using it as a stand on kitchen counters when cooking that the cover itself was getting the screen smeared.
I've since bought her the Belkin Cooking Stand/Kit and we're both happier.
