I like to use up my left overs from the front. If I dont' have enough left, I like to use this method:
http://www.maryquilts.com/off-center-4-patch-backing/
This lady has alot of free patterns. It is not a commercial site. She just asks that you not sell her patterns. I only use solid fabrics if I'm REALLY confident of my quilting design. The bobbles seem to show ALOT more on a solid!