For those self-distributing their film: this is a bat-shit crazy idea that surfs the wave of goodwill that happens at the end of a showing when you do a filmmaker Q&A.
Here's what you do: Give cash away at the beginning (your cut of the ticket sales) in denominations that don't match what people payed ($1 in some cases, $50 in others) -- and then ask them at the end to return the cash if they liked the movie, and add a bit extra if they really liked it.
Or they can keep the cash if they didn't and if they don't support the film.
Not quite a money-back guarantee but based on one guy's experience (for a theater show), people clearly are more generous than not. He actually earned more this way than he would have through ticket sales alone.
It makes sense: people don't want to be seen as jerks and want to support indie artists and performers. I would only do this if you're doing a Q&A at the end of the film, so they can see you're a flesh and blood person, with feelings and hopes and dreams.
Give it a try and let me know how it goes.
BTW, I doubt it works if you don't show up. Without the pressure of the filmmaker (or someone from the filmmaking team) standing right there, people are more likely to just leave after the movie is over and then that's that.