I've only participated in 3 book signings so far, and they went well. I think I've learned a lot with each one. One thing that is critical, I think, is the freebies -- you've got to give something away even if somebody doesn't buy from you.
That's why I use my recipe cards (see http://www.jayellwilson.com/recipes.htm for examples that I kludged together from the real ones I have made).
I've found that it's best to hand out the cards -- if I stack 'em up, people assume I have cooking as a theme in my books. Au contraire -- I am a lackadaisical cook at best and the recipes I provide are simple, easy, and guaranteed yummy, the kinds of things my heroines all cook (as evidenced in my books).
I'm still trying to find that one promo thing that declares, "This is a J L Wilson book." The cards are nice but not quite what I'm looking for.