But screening/marketing to clients is like making a decision based on credit.  You want to pick the clients who are the LOWEST RISK to be fucked up.  If a guy is 50, with a family, has a stable occupation, positive personality, and is cooperative during screening, I'm picking him over the younger Ebay seller every time.

I've been an independent provider for as long as you have, and I agree with Superstar!. I've also never had a NCNS, or even a last-minute cancellation.

Usually, there are indicators of flakiness...a last-minute request isn't necessarily one of them, but if I get a general vibe...I don't know, sometimes you can just tell when a guy isn't serious. If I get that vibe, I usually don't make the appointment, as they usually have issues (disrespect, asking questions clearly addressed on my site, like 'What are your rates?', just being generally annoying) that go beyond flakiness.  

Here's what I do with all appointments: as we set the date, I'll tell him verbally that I'll be confirming, and I'll also say when I'll be doing it... day of, or the night before, or whatever. Then, at the appointed time, I send an email to confirm. If I don't hear from him by x time, I'll say, 'Hi, haven't heard from you...just want to confirm that we're still on. If I don't hear from you by 5PM, I'll assume you've made other plans. Have a nice day." Then if I don't hear by 4 or 4:30, I'll fire off one more, just to double-check. That way, if he does email me 15 minutes after the appointment is supposed to start, asking where I am (that has never happened), at least my a** is covered.  

Short or one-line emails, not reading the site, changing times, suddenly asking questions like 'What is your rate?' that are addressed on your site or elsewhere...and definitely cancelling 2-3 other times...is a clear indicator of a flaky guy. I wouldn't have booked him, personally.