Don't. It doesn't hurt to lightly dust with a Swiffer or very lightly with a can of compressed air... but serious cleaning can take away the value of the object by removing some of the signs of age that collectors love. Even worse, you could damage the artwork or discolor the piece. If you must, you could try a damp cotton rag. Dab and pat at the dirt to lift some of it away. Be very careful not to rub or scrub, and test in an unseen or already damaged area.