I have a sticker with my book's comical Spider on it that I pass out to all employees to wear during my book signing. If you have a public address system, announce that the author is in the store, where in the store, and mention the book several times during the allotted signing time. Prepare an attractive display in the middle of customer traffic. Have the author sign there. Last but certainly not least, please, pay attention to the author.
Little considerations do go a long way.
And, after all, we're doing it for free.