Let your agent take care of any problems that you are having, or anticipate. Call your agent before you call your publisher. If you have a co-author, always have a written collaboration agreement. As closely as possible follow the outline you have submitted in your proposal. If your book is going to deviate substantially from the outline, check with your editor.
Publishers plan the book's price based, in part, on the book's length.
If possible, submit the first few chapters early in order to get feedback on your style. Specifically ask your editor to review your chapters, and make the request in writing. It's easier to make a few changes as you write than to have to make a large number of changes later. Ask your editor when you can expect the edited manuscript back, and warn your publisher if you are planning a trip between when you deliver the original manuscript and when the book will be published. After you deliver the manuscript you will need to review and edited manuscript, and answer your editor's queries, a process that can take anywhere from several hours to a week.