There are two commonly recommended methods for cleaning oil paintings: using either saliva or soft bread, to gently remove accumulated dust, dirt, stains and other imperfections. Although it sounds odd, the use of saliva as a chemical cleaner in art restoration is well-known in museums around the world. The bread method is also common but is better for removing dust than for cleaning stains.
To clean the surface and remove stains or residue, use cotton swabs lightly coated with saliva. Be sure to work in small sections, no more than 1 square inch at a time. To avoid damaging the paint, dab the cotton up and down gently to remove unwanted imperfections. Once a swab is dirty, discard and replace it. Do not overwork a dirty swab, as it only moves dirt around instead of cleaning the painting.
To remove dust or dirt, use a loaf of doughy white bread. Slice the bread, and pull out the middle, discarding the crust or any hard outer sections of dough. Use the dough to dab and brush gently along the surface of the painting. It picks up dirt and dust without damaging the paint itself. When a slice is dirty, discard it, and continue with a new piece. Once you are finished, use a large soft brush, such as a paint brush, to softly brush away any crumbs or leftover dust from the surface of the artwork.
These methods can be used safely in the home for private art collections. However, if your artwork is of high value, vintage, or antique, or you have any additional concerns for its integrity, consult a restoration professional before doing any work on your own.