I have found COLOR BOOKS very useful. There are so many nice elements that can be traced out of a color book and used in a quilt. I have a couple huge 1 and 2 inches thick color books that I use whenever I want something kind of new or different or need a "special" shape for the quilt I'm working on.
K3N's suggestion for using things laying around the house is a good one. I use Cool Whip lids for alot of circles but I also have an ExActo Knife that works great to cut shapes out of the CW lids then transfer the "pattern" to your quilt with the use of chalk or disappearing pencil. You can also cut out "tear drops" or flowers from the CW lid to add extra interest and detail.