Glad-that-you-asked-let-me-tell-you-what-I-think.
* We held it in a public place. Three other cities tried to clone ChiSec, but held it at office buildings. Think about a first-time guest: there's no risk of embarassment walking into a bar and not finding anyone. But knocking on an office door at 6:30PM? None of those meetups lasted more than 2 dates.
* No RSVPs. I was going to go to a Chicago Ruby meetup a few months ago. Then I saw the RSVP list. A few mojitos in my living room started to look a lot more attractive. Not to mention: it's 6:15P the night-of; what, you don't want the people that just found out to show up? Every time I've ever had people over, it's always the ones I rope in at the last minute that make the party.
* No presentations. Find the best presenter you possibly could, and at most 60% of your regulars are really interested. But next month, you're not going to get the best presenter you possibly can; you'll succumb to the Unix User Group Syndrome and host a talk on "Systems Administration Programming in Awk".
* No sponsorships. We get asked. Of course we don't. This probably doesn't even bear explaining.
My other suggestion is, go quarterly, not monthly --- at least at first. If you do 4 back to back monthly meetups, and attendance starts at 15, dips to 8, then 10, then 8 again, you've set a bad precedent. If you can build up a quarterly meetup and get 20-30 people in the room... well, if you pick the room right, 20-30 people actually feels like a crowd.