I use a product called "Lift Off". I've only found it in Ace Hardware. It comes in a white spray bottle and there are two versions -- one for water soluble stains (food type or biological stains like blood) and one for non-water soluble stains like oil and grease. Just a little sprayed on with a rag and it will lift the white scuff marks from dock bumpers. Use it in a well ventilated area, though -- the vapors are pretty tough!