Understand that the majority of people attending open houses are worried that the agent is going to be aggressive and try to get them to do something they don't want to or aren't ready to do. Many already have relationships with other agents. I truly feel that it's as important to respect the customer's feelings as it is to engage them as business prospects.
People often attend open houses as a form of entertainment and time passing. They may visit for a variety of reasons and are sometimes there not for themselves but other people, friends or relatives. It's very important to get a read and respect people's reasons for being there.
Appreciating all of this, one of the approaches I take is getting totally prepared for it and accepting the fact that the chance of selling this home via this open house in very minimal. BUT...they may be interested in what I know about other opportunities. To this end I always enter an open house situation with information about several other best buys, and two separate lists of area short sale and foreclosures or a real estate tip sheet.
I want the customer leaving the open house feeling that I know the local market and the business and feel comfortable enough with our interaction to want to do business with me...they always leave with valuable information including how to be in touch with me.
Honestly, my philosophy is that if they are happy with what they see, are impressed with the information I have presented and like how I interacted with them, they will offer me their contact information.