Formatting. Having a manuscript isn't enough. Having a manuscript that looks nice will do much better. You want to make sure that you format your manuscript in the correct way. This includes having a title page, a copyright page, a table of contents and page numbers all the way through the book. You will also want to make sure that, if you have chapters, they start on the odd number side of the book.
Copyright. Don't do anything with your manuscript if it isn't copyright proof first. All you need to do is send it to the Library of Congress, fill out a small application and pay a fee. You will, in return, receive a certificate that claims you as the rightful owner.
Edit it once more. No matter how many times you have edited your manuscript, do it again. You want to make sure that you have everything polished, in order and said exactly like you want. Remember that once it is submitted and published, you can't take anything back until the second edition.
Say it once more. One of the requirements for submitting a manuscript is having the query letters, synopsis, author biography and selling points of the book. While many will ask for this specifically in their publishing requirements, it's always good to have what you need on hand.