For me:
First I think about how the quilt will be used. Will it get a lot of lovin/abuse? I will use a strong backing like minkee or fleece.

If it's a nice size bed quilt, wide backing fabric. Since I'm not the best quilter, I like a busy print to hide my stitches.

I also like to use flannel - but I wash it first to get it to shrink as much as possible. Do you know - some stores sell wide backs in flannel!

I don't like plain fabric for the back since I like to hide my stitches. If you are a talented quilter a plain fabric will showcase your stitches.