Over the years of being in the interior design business, I have found that clients are intimidated when it comes to artwork - not just selecting art, but hanging art as well. People tend to hang art either too high or too low. I come across this more often now that I'm living and working in Florida. New construction here usually consists of houses with soaring ceilings. Clients feel the need to fill the height by hanging art nice and high, which is not the right answer.
Artwork should be hung at eye level, where it can be observed and appreciated with ease. I like to have the center of each piece at approximately 5'6" from the floor when hanging on a blank wall. This height is a great average eye level for most people.
If a piece is being hung above a sofa, console, dresser, etc, I like to have the bottom of the art hang about 4 inches from the top of the furniture. This will keep the piece connected to the space it's in...you don't want to leave too much negative space.
Groupings are little more complex and required a bit more thought. First, lay the grouping out on the floor to get an idea of how the pieces fit together. When hanging a grouping, have the pieces in the grouping as close as you can...anywhere from 2 - 3 inches of blank space between, depending on the sizes of the pieces. This keeps the grouping close together so that it tells a complete story.
Hope this has been helpful! You can always contact me for any help you need with artwork (or interior design service)!