An advance reader copy is a prepublication version of a book that the publisher sends to reviewers for free so that the book can garner reviews and "buzz" before the book is released to the public. Publishers tend to be guarded when it comes to releasing advance reader copies---they want to ensure that the copies will be read, reviewed and publicised---not sold or simply filed to a collection. You're not likely to get one of these copies unless you're an established and trusted book reviewing service, bookstore or library.
Establish a niche for your reviewing business. For instance, you could focus on romance novels or finance-related self-help books. The tighter your niche, the more likely a potential publisher will look at your reviewing service as a value to its promotional efforts.
Develop your reputation as a reputable reviewer. When you're getting started, purchase books, read them, and build up your library of reviews. Make sure you take your time to read every book carefully and write a thoughtful and thorough review for each one.
E-mail or call publishers of books in the genre that you chose in step 1. Start with small publishers, and work your way up to the traditional publishers. Give information about your reviewing service along with a link to your library of existing reviews. Leave your full mailing address where the publisher can send you advance reader copies.
Contact the publisher and identify yourself as a bookstore or library representative (preferably the owner or decision maker). Give your store or library name and offer a website address where the publisher can go to find out more about your organisation.
Ask the publisher to consider your store for advance reader copies. Provide information on the volume of sales or interest that books of the publisher's genre receive in your store or library. Explain that you want to review the book for a possible order in the future around the time of final publication.
Provide the publisher with your store or library's contact information so that you can receive advance reader copies of new releases. Ask the publisher to also send an order form, purchasing agreement and a catalogue to make it easier for you to order books in the future.
Some publishers also hand out advance reader copies at book conventions. Set up a table at these conventions advertising your book review service, store or library. Avoid writing overly negative reviews because that could scare potential publishers away from sending you an advance copy---if you really don't like a book that you've purchased when you're still developing your reputation, don't review it. Do not resell your advance reader copies. If you do and a publisher finds out, you could damage your reputation and reduce the chance that publishers will keep you in the loop when new books are released.