mostly I watch for sales - really good ones say of about 1/2 off or so - and then I try to find a print that I think will hide whatever screw-ups I make while quilting; or else I try to find a particular fabric to go with a particular quilt top. It took me some time to find just the color of pink that I wanted to put on the back of a 'vintage' repro yellow quilt top I'd just finished. I found some for about 4 dollars a yard and bought several yards. I have a particular 'stash' of backings I have bought and set aside for just that purpose.
Other times I cut and paste you might say, with what I have left over from the front. And I've also used just plain old tan muslin on the backs of more than one quilt. I once used a toile for a special quilt I made for a wedding. The toile was made for that I think as it was very wide.
Then I have made special labels for the backs and incorporated them into a sort of patch work type backing. It's fun sometimes to try to figure out how you can make your quilt more interesting by thinking about the back of it.