Just some thoughts that come to mind (some will only restate and thus emphasize what others have suggested):
Have a line or two to get a passerby's attention. More than just "Hello."
"What do you like to read?"
"Know anybody who likes to read Fantasy (or SF or horror, etc)?" --depending on what you write.
are some generic starters.
Know a bit about what some of the other authors are selling. If you write horror but they like to read fantasy, point and suggest they visit such and such author's table. Some authors at events are real cuthroat while others will reciprocate. But if you're not going to attract that reader/make a sale, what have you got to lose?
Watch a potential reader's eyes. You can tell if they're interested or just being polite. Don't talk the whole time. Offer/hand them the novel and let them look it over. Have a twenty or so word pitch or description of your novel. Several depending on the angle you're guessing the reader might enjoy-connect with. Then a more detailed explaination of the potential customer asks for more details.
Depending on the crowd--likely visitors, you can have a sign-up for a drawing for a gift card (like an itunes card, or a gas card or something with $10.00 or whatever on it). Put names and email addresses on a slip of paper in a box. (plus, you get an emailing list that way). You can offer two chances for someone who purchases your novel, one chance for someone who just signs up...
I worked with my publisher (on this before hand--established a contract/naming agreement) and for my second novel had a "Name a character in the next First Civilization's Legacy novel". It was similar to the card drawing above, but over a series of book signings, people who signed up had a chance, even if they did not buy a copy of my works. It drew interest, and I could ask people going by, if they'd ever named a character in a novel.
Have a bookmark or business card that you can give to people who might pick up your work later (sales seem to spike a bit after an event). People often have only so much money.
Who is taking care of the sales? You or the event? Be sure to have enough money and the right type for change. If your novels sell for $10, it's easier than $9 or $12.95. Sometimes if a novel normally sells for $11.99, discount it to $10.00 makes a little difference. I am guessing the local municipality won't require a vendor's licence from everyone there if each author is responsible for individual sales?
Have post it notes, and ask the person to write their name (or the name of the person) you're signing the book to. It helps if you have trouble understanding the individual, and saves having to ask how to spell a name multiple times. Kris vs. Chris, or Johnathan Smith vs. Jonathan Smyth.
Have a few pens available.
Have a small stand to set at least one of our novels so that it isn't laying flat on the table.
Bring a table cloth is one isn't provided for the table. Solid color might be better.
Something for you to drink while at your table.
Don't hesitate to just chat a bit with someone who stops by--mabye about a favorite author of theirs or movie or the like. Be a person. Sometimes those individuals will circle back around. Many times people have only so much cash and are previewing before making a decision.
Smile. Be polite and professional.
Banners or posters etc. can be good depending on the space you're allotted. Some places have authors share a 6' table. Others don't.
Also, note that a place with authors gathering is a opportunity to network--future reviews, blurbs, advice, information on other signing events, etc. Here a business card too can be handy.
Good luck, have fun, and learn from the experience!
