(though probably adaptable to many other groups):
Debates seem to get more of a turnout than lectures.
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.
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.
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.
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.
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.
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.