One of the most useful things to tell people at a meetup is when the next meetup will be. This requires you to know when the next meetup will be. If you don't know when the next meetup will be then the next best equivalent is telling people how you will announce the next meetup. The summary of this post is that if you want to convert occasional attendees into regular attendees, I think you should have a habit of always knowing the next meetup when you run a meetup or at least should have a copy of some kind of mailing list on you.
You have now read the basic point of this post. If you want to read on, cool, lets talk about implementation details for a bit.
I grew up attending a small town church. I have not been back to that church in over a decade, but I can tell you when their next meetup is: it's next Sunday at ten. That is an incredibly powerful tool for return attendance. You don't need to be quite that regular (though note that the NYC community attributed some of its success to one person committing to be at the same place at the same time each week, ctrl+f for "the brilliant move") but one time you know attendees are listening to you is at the meetup. Why not take advantage of it?
Assuming you run good meetups that people enjoy, then as they're putting on their jacket and getting ready to leave they're in a good mood. This is an excellent time to prompt them to consider coming back. "If you had fun, we'll be here two weeks from now doing it again. Hopefully I'll see you then!" The term for this is a call to action, and they're widely used because they work.
If you know what your next meetup is about, then you can catch some of their interest. "Next month we're doing a trivia game with calibration estimates. It's usually pretty fun!" If they're the kind of person who likes calibration scores or trivia games, now maybe they're more looking forward to it. I have had times in my life where the thing I was most looking forward to at the end of the week was seeing some friends at a meetup on Saturday.
Plus, the sooner you tell them when the next thing is, the less likely they are to double book themselves. There are lots of cases where I find out about some event only after I've already made plans for that time, and then I can't go. If I'd known ahead of time, I could have scheduled things so that they didn't overlap. Since you usually can't schedule your meetups around individual attendees schedules, your other option is to let them know first.
I have a lousy memory. It's easy for me to forget how to get in touch with someone, though the internet makes this easier. If someone wants to come back for more, make it easy for them by putting them on some kind of announcement setup (Facebook groups, email lists, Meetup.com, whatever you use) and then using it to tell them when you're doing more. Let it remind them.
By the way, you can both tell people about the next meetup during the current meetup and also tell people about whatever electronic notification system you use. You can do both! Maybe they can't make the next one but they'll show up after that.
You may feel like you're bothering people too much. It's possible you might actually bother people too much! I am going to state here that the correct amount to bring up the next meetup and your mailing list is about once a meetup if it's small enough everyone is in earshot at the same time, or once a person if you have reason to believe you're getting each person once. An easy way to offer it to each person once is to do it as they leave, since they're usually only going to do that one time a meetup.
Lots of people these days have smartphones. Instead of having them handwrite their email address on your signup sheet (which assumes you will be able to read their handwriting) or having them type something into their phone, consider making your signup sheet a google form accessed via a QR code. Turning a link into a QR code is pretty straightforward.
If your meetup regularly shares members with other meetups, consider sharing a signup form and letting people mark which groups they want to subscribe to. At an ACX Everywhere event it's a reasonable guess that lots of people might want to hear about local LessWrong or Effective Altruism events as well.