I tend to use fabric that I have had forever and have no earthly idea why I still have it. If it has the colors of the quilt top and compliments the quilt top then usually that is it. A big part of choosing is if I want my quilting to show or be invisible. For example, the quilt I'm doing now is primarily aqua so I selected a floral, pale, with shades of aqua as the backing. I am quilting using a varigated aqua thread and a geometric pattern, the back looks very pretty so far. I recently made a quilt for my son using his college logo fabric, with musical sashing as he played in the marching band and bordered it in black. I choose a red tartan plaid for the backing as to me it was in keeping with the fact that it was for a man. Then when I quilted it I used a gold thread and it made the back really pop. So I guess picking a backing is really just doing what looks good to you and compliments your beautiful work on the top. Oh, quality of the fabric is super important to me. I will not use a flimsy fabric for the backing. Hope this helps you a little bit.