Don't tell anyone, okay?
I've found standing in front of the table is best. I think if you read about body language, behind a table or desk is always considered more intimidating. I hold my book up in front of me like the perfume salesladies do--that sort of warns folks that I'm hawking my book. So they know I'm not trying to get their cell phone biz. Then I say, "May I tell you about my book?" Or better yet, "I'm a local author signing here today. May I tell you about my book?"
Usually, they hesitate, then say, "Uh, okay." If they say, "No." I thank them politely and step away. Often they come back by. The trick is to be friendly and CALM. I've found that sounding very, oh, confident but not excited works best. (It's hard because your heart is pounding.)
If they have a child with them, I ask the child, "Can you guess how many words are in this book?" They never can, but I give them a bookmark for trying. Parents appreciate anyone being nice to their kids, and they love the fact you are giving their child something to keep him/her occupied while the parent shops.
I'm averaging selling 8-10 books an hour this way. I keep refining my pitch. This is the best training for getting to know your audience ever. And the booksellers start to get really excited when they see how your book is coming through the check out line.
One of my friends said, "I could NEVER do this."
Yes, you can. If you believe your work has value, you can do it. In the beginning, you'll feel absolutely sick at your stomach with terror. You'll search their faces and see NOTHING and think, "Oh, my God, why did I ever sign up for this? They HATE me. They think I'm stupid." Then to your surprise they'll say, "Could you sign that?"
It's just amazing. I figure I sell one out of four or five people I approach. It's strictly a numbers game. But one I'm willing to play to move my books.