Audible has a policy where they will allow you to return any audiobook within 1 year for your credit back. It's a no questions asked policy from what I understand.
So usually I'll just use up my credits as they come. Then when I want to listen to something new and I don't have any credits left, I'll look through my library and see which book I didn't really like that much and exchange it for a credit to listen to something new.
So it's basically a Netflix for Audiobooks.