Most often when I am creating a set I am definately driven by color and fabric rather than say a certain theme or using a certain pattern. For example, for Sugar3's Fairy Sweet launch I knew that I wanted to incorporate hot pinks and reds into the set. I then went on to decide on a theme and design.
Here are some "before and afters" of some of my current sets.
A little peak behind the scenes. :) You can find these two sets here on ebay.