Use a rug to define or separate areas, such as seating or dining areas, and foyers.
If you use more than one rug in a room, make sure they are not the same size. Same sized rugs can cut your room in half visually.
When using more than one rug, it's better when the rugs complement each other in style. If not, you could end up with a jarring or unpleasant effect. Too many "warring" patterns in a room will do away with any sense of harmony.
Use a favorite rug as the basis of the color scheme in a room. On the other hand, if you add it after you have your furniture in place, you can use the rug to accent or tie in your existing colors.
Use a rug as the focal point of your room, as rugs can make a huge impact. To do that you can create contrast by painting your walls a hue that echoes one of your rug's accent colors.
Use a rug as a wall hanging for another way to create a focal point.
When you buy a room size rug, subtract 3 feet from the length and width of the room. Leaving the floor bare at the edges makes it look a little larger. Also, when laying a rug under your dining table, leave at least 24" of rug from the edge of the table out on all sides. This enables the back legs of the chairs to stay on the rug, even when pulled out to let someone sit.
Runners should be 4 inches narrower than your hallway and 18 to 24 inches shorter. Make sure they are wide enough to accommodate both feet on the rug when you walk.
Always use a rug pad to keep your rug from slipping or creeping. The rug liner should be appropriate for your floor type.
sourced from About.com: article by Abe Abbas
Dr. Khosrow Sobhe (Dr. Kay)
Certified Rug Specialist (CRS)