Right after I borrowed my friend's backpack to try out the other week one of my children put their hands all over the windows. Having given back my friend's backpack, I gave the windows a go over with the 'Flash car washer', which is in effect a hand held DI unit that fixes to the end of your garden hose, and they came up a treat. I didn't even have to use the soap. Just rinsed with tap water, sponged over them then rinsed with filtered water.
I wouldn't advertse this to my custies though (because you can pick them up quite cheaply now). It's not much good for doing the tops though unless your a bit of an enterprsing character in which case it could be done.