Idea architect Jeffrey Cufaude offers a bunch of great, easy-to-implement ideas on how to make it easier for your attendees to connect with each other, from including an "ask me about" line on the name badge to organizing registration around things that might actually spark conversation, such as regions. Why aren't we doing this?
P.S. I just found this guest post by William Thomson on the Event Manager's Blog on how you can make your meeting's networking opportunities more valuable. There's some overlap with Jeffrey's ideas, but some different takes as well.
What works best in your experience? The best networking (and I really don't like that term much either—it makes us sound like a bunch of computers in search of USB ports or something) enhancers I think are just providing time, proximity, and social objects that help get conversations started, as opposed to merely getting business cards to transfer from person to person. This is at least in part why I think more and higher quality relationships get built at small meetings than big conventions—you are with the same people for the length of the meeting, in sessions, during meals, and during down time, and really do get to know each other on a deeper level than job title and elevator pitch.