Do you have bookmarks to promote your book(s)? They are inexpensive yet, when designed correctly, an attractive promotional tool. If you have more than one book, use your bookmarks to advertise all of your books. For example, I have six books in one series. Of course, within each book, I've got a listing of all the other titles. But what happens when you come out with a new book and the others in the series don't list it? This is where bookmarks are really handy. They are inexpensive to print and can be updated easily. I slip one into each book that goes out (push them snugly against the spin). That way, I know the eventual buyer will be alerted to all of my other books. This is particularly useful when the books are going out to places where I can't include flyers. (Think Amazon, Baker&Taylor, etc.) A few may slip out during the handling process, but that's okay. I believe most make the journey and help promote my books.
Bookmarks handed out at book signings work well too. Customers frequently act as if they're getting a nice freebie. If somebody stops at your booth and doesn't buy a book, hand them a bookmark before they leave. They may just change their mind later. Be sure the necessary contact information (company name/address, website, phone number, and email) is included. You may even want to consider a special offer code on your bookmarks for, say, free shipping, 10% off an order, etc.