How to design a book cover

This post by Kathryn Rosa Miller is a fantastic, in-depth look at the process of designing a book cover, from developing the initial concept to final typography.

So! The bad news is your current cover is not on the right track but the good news is you’ve got what sounds like a cracking premise/hook which ought to get plenty of attention as soon as people catch the idea.
So the job of the cover is to make that as clear as possible.

— Cover advice: The Last Girls Standing

The rest of her site is well worth a look, also.