The common perception among many real estate agents is that the only reason to do an open house is to make more contacts, and some agents will say even that doesn't work.
It's highly likely that your open house probably won't be the only open house that visitors get to that day. You can make sure, though, that even if your listing doesn't meet their needs, your name remains in their minds for a long time, what my marketing guru calls "Top Of Mind Awareness."
What I do to ensure that is to create an "Open House Survivor Kit." I keep them hidden in the closet at the front entrance, and as people are leaving, I give them one.
What it is, and the cost:
common lunch bag, except that I use the white ones rather than the brown ones. They cost 1¢ each ($4.99 for 500 at Costco)
business card (4¢)
neighborhood search game, created in Excel. (1¢) San Diego's 101 neighborhoods are hidden in a mass of letters. They have to go to my web site to find the answer, which drives traffic to my web site. Do an Internet search for "games" and you can find lots of free sites that will instantaneously create games of various types, such as word searches, crossword puzzles, etc.
pen with my name on it (19¢)
individually wrapped snack, like peanut butter cheese crackers (30¢)
3-5 individually wrapped candies (5¢)
CD or DVD with documents on it containing tips and tricks for home safety and maintenance, provided by my choice of home inspectors (free)
a $20 discount coupon on their home inspection, provided to me by my choice of home inspectors, and good advertising for him) (free)
label for the bag: "Open House Survivor Kit, Compliments of Jim Frimmer, Realtor, Century 21 Award" - done in a scary "survivor" font (5¢)