I've said it in other threads and I'll say it again here; make them put some skin in the game in the form of a 10% deposit via PayPal or something. Tell them it's only refundable if they meet as promised and then decide they don't want it. Once they know they have to put a few $$ into it, you'll get far fewer no-shows, and only the serious ones will go through with it if they want you to hold it for them. The ones who aren't sure won't follow through on it and you won't be out any time, gas or $$.
It's just too easy for someone to claim they're going to show up, and then leave you hangin'. I think some people do it for sport. Forcing the issue will weed out most of the assholes like that.

