Charge a Refundable Attendance Fee
The Problem
Tons of people RSVP for your free event, and you prepare accordingly - you order enough to feed them, and book a big enough room to fit them. But many of them never show up. You eat the extra cost, and the event starts off as a disappointment.
The Pattern
When people sign up, charge them a small fee, and refund it when they show up. Your event will still be free, but a) people will think twice about registering, and b) they’ll be more likely to attend, to collect their money.
Meetup and EventBrite both make this fairly simple. You can have a greeter at the door with a tablet, and refund the attendee’s fee as they walk in.
Common Variants
You can also give them the option to collect their refund: if they prefer, they can donate it to the group. If the fee is small enough, you might find that enough people people prefer to do this, that it can help with your costs.
Caution
This probably works best for events that are expected to be free to attend, like an unconference. If your group normally charges for events, this might undermine that expectation.
If you’re struggling to get people to RSVP at all, you probably shouldn’t use this pattern - it basically discourages people from RSVP-ing. This pattern is for when the people who RSVP don’t show up.
Example
Up to three examples should suffice.
A First Example
Product Camp Boston had this problem, and solved it with this pattern. The description is taken directly from their story.