An oil painting can be cleaned by rubbing the surface with a plain piece of white bread, then dust off the crumbs. NEVER try to clean a watercolor or pastel painting.
Source: I paint and have learned these tips in the course of my studies.
By Maya Lee from Redlands, CA
Cigarette tar can be nearly impossible to clean from an oil painting yourself. It may require professional cleaning. This is a page about cleaning cigarette tar off an oil painting.