One of the most effective ways to encourage participation at a large meeting, such as a Town Hall or a retreat, is to use electronic voting pads and software. With it, those who attend can vote anonymously and see results instantaneously. (I have teamed several times with Express Interactive Solutions.)
For example, at one retreat I facilitated several years ago, we asked the lawyers put themselves in the shoes of their clients. They considered ten traits of Legal Team effectiveness, and used voting pads to vote on how important they thought clients would think those traits were. We compared those scores to what the lawyers thought of the Legal Team’s performance on them. [For more on retreats, see my March 31, 2005 post on Morrison’s Rule of Three.]