Pay attention to the author. Ask if everything's okay or if the author needs something. Get the author something to drink if you have it. Prepare an attractive display in the middle of customer traffic. Remind customers frequently that you have an author signing in progress.
Authors are also great readers and love bookstores, and they like to help out where they can.
Authors also like to help out schools and other organizations that might sponsor signings. What authors don' t usually do signings for is money. Authors also talk to each other and tell each other where they've had good and bad signing experiences. There are some bookstores and schools that I'll probably never go to again. I hope your signing is in the latter category.