One common error people make when hanging photos is placement - they either place them too high or too low.  Your best bet is to hang photos at eye level, putting the center of the image about 5 1/2 or 6 feet from the floor.  If you're hanging pictures above furniture, a good rule of thumb is to place the bottom edge of the piece about 6 to 8 inches above the top edge of your furniture. 

Hanging artwork securely is also important.  Heavy pieces must be secured to a wall stud, which you can find by using a stud finder.  You can use a basic model or try the Bulls Eye Auto Leveling Laser Line and Stud Finder, which not only finds studes but also makes it easy to be sure your picture is level.

Be sure to buy a picture hanger with a weight rating appropriate to the art you are hanging.  It is best to use two hangers where possible.  This makes it easier to keep the item level and helps to prevent wide objects from leaning off the wall.

Before you put hammer to nail, cut templates of your pictures out of brown paper and tape the "art" to the walls so you can see the placement.  this will help you avoid putting a bunch of holes in your walls!!