Make it short and brief. Give them several versions because they usually announce that you are there several times. If you 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.