I like to use leftover fabrics from the front on the back, or coordinating fabrics from my stash. Sometimes it only takes a few of them, other times it takes a lot. I figure out how to piece it based on how many fabrics I need to use to cover the back, and the size of those pieces. Here's an example where I had a lot of fabrics and not any really big pieces -
http://www.quiltingboard.com/picture...rs-t82165.html
I usually use either EQ or pencil and paper to figure out a design. You piece the back just as if it were the front. I quilt these on my longarm, no problems with the extra seams (so far at least). If there are just a few seams on the back, I place the quilt on the longarm so that most of them run horizontally.