Say they make an appointment a week in advance, for Friday at 5PM. Once we've set everything up, I'll send an email saying 'I'll be in touch by Thursday afternoon to confirm.' On Thursday afternoon, I send the email. Only one time have I not heard back. In that case, I emailed him again that evening, saying 'Haven't heard...if I don't hear back by tomorrow 12PM I'll assume you've made other plans. Have a nice day.' If he still doesn't reply, I'd fire one off at 11:00AM just to give him one last chance (and cover my ass).
Even if the appointments are booked with less lead time (say 24-48 hours), make sure they know that you will be in touch to confirm...and, if they don't reply, then no appointment.
If these are same-day or especially last-minute requests, there's really nothing you can do. A high percentage of last-minute requests are made on impulse, and those guys do tend to be flakier. So if you're able, you could simply not accept same-day requests, or accept them very selectively...for regulars, or guys who sound like they are very serious about actually seeing you.
Also, if the guys seem really indefinite, or if they haven't done research on you (asking for rates clearly posted on your site, changing times constantly, setting things up over multiple emails in text-like fashion rather than being more efficient and doing it in a few emails - that can indicate ambivalence about keeping the date)...those are dates you might only want to take if you're already at your incall location and don't mind sitting around waiting for a date that might never materialize...
Hope this helps!