Don't use a metal one, you run the risk of causing damage to the upvc, or if you do use it, make sure you angle it down (hand closer to the frame) use a plastic one just to get the bulk off, then a white scourer will get the stains off.
Anything more difficult to get of ie stains that you can't remove, I can give you two options and I have used them both with a great success rate.
1. Wet & Dry paper (Halfords) this must be the finest grade you can get, I think its 2000 grade, wet it and the sill and rub along the length of the sill and not across it. wipe down afterwards, if its for a regular custy, just rub a little wax/car polish on it and buff up. (this tip was given to me by a plasic sergeon)
2. A little bit more aggressive this one and good for dis-coloured upvc or paint left on it.
2 cloths required (paper towels work best) and work quickly.
Soak first cloth with cellulose Thinners and wipe offendiing area, quickly remove residue cellulose with a clean dry paper towel, you can buff it up with your own cloths afterwards if needed.
This tip was given to me by a painter.