1. 57-60 inches on center is a good place to start when establishing the height of a piece of artwork. Keep in mind that this is just a guideline, it all depends on the overall design of your room. Start there and adjust to what looks comfortable or interesting to your eye. Ceiling height, size of furniture, intensity of color and amount of pattern all will effect the height and position that looks right. In the wonderful example shown above, from Living Etc., the highly graphic butterfly piece can stand out and be noticed in relation to the bright red sofa and patterned wallpaper due in part to its off-center position.
2. Generally, the scale of hanging artwork should be in proportion to furniture. Artwork should usually not be longer than the width of the widest piece of furniture. A guideline to aim for is that the artwork be approximately 75% of the furniture width for a pleasant proportional relationship.
3. If hanging multiple pieces in one area, think about the general direction of your grouping and the overall feeling or impression you wish to promote in your particular space. Horizontal arrangements of frames generate the feeling of expansiveness and width, whereas a more vertical layout gives an impression of height.
These are the basics - please add any tips that you have found helpful in the comments below...thanks!
Image: Living Etc., March 2009