(though probably adaptable to many other groups):
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Debates seem to get more of a turnout than lectures.
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If you can’t set up a head-to-head debate, for instance because local professors (see below) aren’t confident that they’ll entirely disagree with the visitor), set up a two-person panel, or a talk-plus-commentary.
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Events that involve a local professor — a debate, a panel, or even the professor’s just introducing a guest speaker — will probably get more of a turnout, because it will bring in the professor’s local fans.
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Publicize, publicize, publicize, using all the tools at your disposal — e-mail, flyers in mailboxes, postings on bulletin boards, postings on class chalkboards, if your school allows that, and whatever else you can think of.
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For topics, the usual sexy ones are good: affirmative action, gun control, abortion, church-state separation, campaign finance, and the like. Other topics can work as well, especially if you can find a well-known visitor who wants to talk about the things he likes. But generally speaking the old standards work well. Even if you feel that not a lot of views are going to be flipped on these topics, sometimes you can succeed just by moving people from unreflective support for the liberal conventional wisdom to a more agnostic position.
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If you want to bring in a relatively prominent speaker from out of town, offer to coordinate with other chapters in your city, so that the speaker can — if he wants to — give several talks on one trip. This may substantially increase the chances that the speaker will want to take the considerable time and effort that modern airplane travel requires.
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Provide lunch — the better, the better. [UPDATE: I originally forgot this one, even though it’s in some ways the best way of boosting turnout; thanks to the commenters for reminding me.]
These are of course all guidelines, not hard-and-fast rules. For instance, I’ve generally preferred to talk about slippery slopes, never with a debater (it’s not a subject that lends itself well to head-to-head debates) and often without even a local commentator, and that’s generally worked out quite well. But I think that most of the time, those guidelines will prove helpful.