Just as a follow-up to Cali's advice on cover art, I found this article over on the Writer's Guide to E-Publishing (a great resource if you haven't found it yet!). One thing I never thought of was a designer's terms of service contract that may or may not allow the use of the cover in other formats, such as t-shirts, bookmarks, banners, and more. Definitely something to make sure you've got covered!
And for more fun, check out Chuck Wendig's blog post on judging a book on its cover. And in his post 25 Things to Know About Sexism & Misogyny in Writing and Publishing (which includes a bit on the dust-up over at SFWA), I was laughing so hard at the link in #12 to Jim C. Hines Cover Posing (and the cover poses) that my husband thought I was crying.
So, yeah, covers are important. And apparently, not to be left for the last minute (much like proofreading!).