As many of the answers suggest, it really depends upon what you’re trying to achieve. Ask yourself what one thing you really want your audience to do after they leave the presentation (or even sooner if you can manage it). You have to spur them to action (even if that action is simply to consider something). Give them a reason to interact.
The best recent example I can think of was a presentation from the guys at moo.com, where they said anyone who came up and said “hi!” and left a card would be sent a code for 10 free cards. They got dozens of cards, and I know of a lot more people who couldn’t be bothered to wait and just used the in-hall wifi to order a full set of 100 cards there and then (that’d be me).