Which comes first: the cover or the final draft? When I write, sometimes I'll start thinking pretty far ahead -- not just about this story, but maybe even about the next story I'm going to write. I've learned that lots of other authors do the same thing. And like me, they have to stop it from happening. Getting ahead of yourself can be harmful to your writing.
Don't get too far ahead of yourself when you're writing a book, because it gets overwhelming. If you start thinking about the cover and the marketing and the blurb while you're still writing the first draft, you're doing too much. Focus on the story itself, and get that perfected first.
Take it one chapter, one page, one paragraph at a time -- don't start thinking about the cover and all of that until the first draft is finished. Because if you start getting too far ahead of yourself, you may start to feel pressured. You may start to rush the story, because you're already so far ahead of it already. You can't live in the future, you can only live in the now...so focus on where you are now.
Get the story perfected and then start filling in the details, because you don't have to get it published right away. Take your time with every aspect of the story and plan it out carefully. Don't be in a rush to go fast, because that results in lower-quality work.