Don't waste your time. Trying to prevent a ladder from being covered in paint is like trying to keep a war hero's uniform from getting covered in medals. Exactly like that, in fact.
Paint stains means the ladder has been doing useful work. When did that fall out of fashion?
Assuming it is latex paint, your best bet is to catch it right away with soap and water. Soaking it in water helps soften the latex paint if it's fairly new. For oils and 2-part paints, fuggedaboutit.