Decorating with pillows is one of the quickest, cost effective ways to change the look of your rooms. You can choose to make your room warm and inviting, add a "pop" of color, add "interest", etc.
To decorate with pillows first you need to determine your color scheme and style. Next, think about layering. Choose pillows in various sizes, styles and complementary or contrasting colors. Choosing a solid or subtle print, can be used as a "back drop" to a busier fabric or to highlight a particularly interesting pillow. Use oblong (12"x22" or 12"x16") pillows for back support or to line the back of a sofa, chair, bed or bench.
Odd number of pillows, formal look.
Odd number of pillows in a symmetrical setting.
An even number of pillows in an informal setting.
Here's a great example of a complementing contrast, mixing traditional and modern.