I lick the sills clean.
Sometimes I put a bit of cream cleaner on my tongue first. It helps to get of the stubborn marks.
I've tried the kitten and also used a baby hedgehog but both didn't last long enough.
Once when I was using the kitten, I thought I'd squashed a fly on the sill as there was a red stain. It's wasn't a squashed fly, it was kitty's eyes popping due to friction.
Also, if you do use a kitten or a hedgehog, or even a new born baby, don't leave them in the bucket over night because they will go rancid and bloated by the morning.