Get an ISBN number. This number identifies your book much like social security is used to identify people. If you're using a self-publishing service, check to see if it offers ISBN numbers. Its generally an extra fee or included in a package. If you're not having your book printed by a publishing service, you can get an ISBN number by visiting ISBN.org.
Add a bar code. According to Barnes and Noble the book industry uses the Bookland EAN Barcode as opposed to a UPC code. If you're using a self-publishing service, see if barcodes are inlcuded in the package. If your book is already printed, and doesn't have a barcode, you can get labels with the barcode. Visit Bowker for barcode information http://www.bowkerbarcode.com/barcode/
Draw up a marketing and promotional plan. Authors can no longer rely on publishers or booksellers to sell the books. Authors need to be a part of the sales process so Barnes and Noble will want to know what you're doing to help sell the book. This can include a website, book signings, media interviews, etc.
Start getting buzz about your book. Send copies of your book to reviewers and other media sources related to your book's topic. Save copies the clippings to include in your submission to Barnes and Noble.
Get CPSIA Certified. Barnes and Noble vendors need to comply with the Consumer Product Safety Improvement Act. You can read the letter from Barnes and Noble plus get the certification form here: http://www.barnesandnobleinc.com/for_publishers/Vendor_Guidelines/letter_to_suppliers_and_manufacturers_cp.pdf
Put together your submission. Package a copy of your book, its price, marketing materials, review clips, a statement about how your book is unique compared to the competition and why Barnes and Noble should accept it, and the CPSIA certification and mail it to:
The Small Press Department
Barnes & Noble
122 Fifth Avenue
New York, NY 10011