When Mark Twain started giving lectures, the book tour of his day, he was extremely nervous about whether the crowds would be friendly or if they'd laugh at his jokes so he paid a few people to sit in the audience to laugh and clap at the appropriate times. Being a humorist of such caliber, he didn't need these guys, of course, but they gave him confidence.
What does this have to do with a booksigning? Simple. Talk to friends and relatives, and ask that they come at a specific time. (Spread them out through the signing.) That way you'll always have people around to attract other people, and you'll look busy to the bookstore owner. You'll also get out folks who wouldn't come otherwise.
If things are going badly, despite all you've done, (it does happen even to the best of us), make certain some of these friends buy your book even if you have to reimburse them later. That's so you look good to the bookseller, too. And for Pete's sake, don't let the bookstore know these people are planted! You may want to come back again.