Michael has posted very good points about fabric.
The first thing I do, is set up for the fabric, observing how that piece plays with light. The color is the frosting.
John Hagan has a good section of how various fabrics behave at a site titled "The Magic of Painting". I believe Virgil Elliot corroborated in the chapter. Hagan is also a very entertaining instructor. Check out
http://www.geocities.com/~jlhagan/
Enjoy.