A number of years ago a friend gave us a great tip that to this day has made picture hanging a simple, pleasurable task. The tip was to always hang your art at 57" on center. "On center" means that the middle of the picture is at 57" (obviously, the hook will be higher).
Interestingly, the 57" standard represents the average human eye-height and is regularly used as a standard in many galleries and museums.
What WE have discovered is that if you stick to this standard, you create a harmony among ALL the pictures in your home, as they will always hang in relationship to every other one. Additionally, we have also found that this helps solve the problem many people have, which is that they hang their pictures too high.
Makes me want to go re-hang all my pictures in my place, cause I know most of them are way over 57".
Also, this 57" also applies to groups of pictures; just think of the group as ONE large picture. After you arrange how you want them all to hang, start with the center picture/pictures and get them at 57" on center. Then surround them with the rest of the group.