Within each genre of artwork, style is analyzed for mastery of technique. Trends in the art world will also affect the value of an artwork.
Provenance includes creator(s), owner(s), past sale costs, and conservation history. Provenance also helps ensure that the item has changed hands in a legal and ethical manner. For example, the Nazi's in World War II stole artwork from Jewish homes that have shown up in art collections.
A rare artwork, especially within a style or by a particularly well known artist, can increase in value.
An artwork published or exhibited can increase its value. For example, If the Mona Lisa was tucked away in the storage unit of the Louvre and not analyzed by scholars on regular occasion, it would have a lower value.
An artwork that has not been cared for will have a decreased value. Major conservation, especially if it alters the structure or appearance of the work, decreases the artworks value.