Forgot this in the progression. This should come right along with starting to consider promotional areas. The book has been sent off. Now, some people sit and wait until they hear from everyone they're queried. What happens to their writing. They stagnate.
Starting the next book is important. A good thing about this is that you have something to take away the pain of rejections. There's a new book to be created. Waiting loses momentum. Selling one book is great but these days, publishers want to know that there's something new in your pipeline. And with the wait time of many submissions, you could have the next book at least rough drafted and sometimes nearly done.
Part of the problem, some writers have is they hate those rejections and they can't think beyond themselves. I always have at least one book outlines and ready to begin when I finish one. For me this always gives me hope. Hope is something every writer needs. You can't think, I've written the next best seller and daydream about what this will mean. The next best seller should be the new one you've started when you've sent off the last.