Astronomy on Tap is still young in Seattle (only 3 events so far) but has already proven to be pretty successful -- we've managed to wrangle 50+ attendees at each of our events!
The astronomyontap.org website lists 6 official locations, but you can get your city added to the ~official~ list (complete with a wordpress page and all) by contacting Emily Rice, who is the current organizer.
As far as organizing the events themselves: We've been meeting at a brewery that also does movie nights (so we knew there would be a projector). Another grad student who was friendly with the owners shot them an email and asked if they would be willing to host an event (with the idea that if it was a good fit, we could make it a more regular event). With the number of people we bring in on a weeknight, they seem to be pretty happy to have us.
The Seattle AoT events are run by grad students (though we feature talks by people at all career stages) but it's been really helpful to have a core group of people functioning as a pseudo-exec board and splitting up the workload --scheduling, advertising, graphics, content generation, etc.
Happy to talk more about what works and what doesn't -- it REALLY helps if one of your team members' S.O. runs a cupcake shop and can provide amazing professional astronomical cupcakes