I like to sketch mine on paper first. I usually measure the top, add however much I want extra to figure out the total size of the back. Once I have an idea of what I want to do, then I play with measurements based on how much fabric I have to work with. Often, I will take the back length measurement and divide by 3. I usually round up/down to the nearest 5 just to make it easy on me. I usually put a strip of something between the smaller and larger pieces. Sometimes I will make a wider strip or strip of blocks, and add a narrow strip on either side, then put that between the smaller/larger backing pieces. Here is a link I started a while back to show some pics of backs.
http://www.quiltingboard.com/picture...s-t148395.html