They will often get busy and forget. Every half hour should do it. If you do lectures or give speeches about your book and are in town for a keynote or seminar that is open to the public, include information about that too. Let the book store choose to include it in the announcement or not. DON'T - Don't just sit at the table they have for you.
Most authors do that.
I always tell the person booking the signing not to worry about putting a chair behind the table. This will always get their attention. Let them know you will be the store's official greeter while you are there. Walk around the store with several copies of your book and introduce yourself to everyone. If those you introduce yourself to show the least bit of interest, hand them a book.