One of the most common mistakes I see when I visit my clients houses is that the art is hung too high on the wall. It makes it seem to me like the room is shrugging it's shoulders or raising it's eyebrows and needs to relax.
The common misconception is that the piece of art, picture, or mirror being hung has to be evenly space between the ceiling and the table/counter/lamp/sofa it is placed above. Maybe if your ceilings are 7'-0" high this works but generally, that type of symmetry is not the most pleasing thing for the eye to look on in the room. If there isn't any furniture below the art, I will try to place it somewhere around 1/2 way to 2/3rd the way up the wall from the floor. Most of the time I see the art hung around 3/4 the way up the wall. Yikes!
As a general rule of thumb I like to hang a piece of art around 6"-10" above a table or sofa. This will also mean that you should have the appropriate sized art. Many work of art I see hung are great pieces but too small for the location.
Here are some examples of properly hung art:
Take a look at your house or office. Is the room shrugging it's shoulders? I think it's time for some relaxation!
Photos from Nancy Pape, Inc. Model Homes