Bringing friends along to the meetup may be a good idea. They don't have to be actively LessWrongian, we have had multiple examples of interesting conversations started by newbies brought along by more active members of the group.
If at a university, advertising to student socities: science, philosophy, maths etc.
I have also been thinking of trying to boost the discussion on the meetup group's mailing list, which may push people to choose to attend the meetup and follow up a discussion. Sequence (re-)runs seem to be a good idea to try.
Also, maybe writing up experiences from the meetups and posting those both on the mailing list, and on the LW discussion board could be useful - people can see some examples of activity and maybe those who previously didn't think it was worth their time will move attending the meetup up on their priorities list.
How to Run a Successful Less Wrong Meetup is a guide that I've been working on, based on lukeprog's instructions, for the last week and a half. As it says in the beginning:
Here's the table of contents:
This is a draft version, so feedback would be most welcome, particularly on things like:
The link above will take you to a Google Docs copy of the document, with the ability to add comments to the draft. Feel free to comment on the guide either as traditional LW comments or by attaching comments to the document itself: both are fine.
EDIT: Here's the most recent version, though without the commenting ability.
EDIT2: The most recent version as of April 11th, with commenting enabled.
EDIT3: First non-draft version; see also this thread.