I've held shorter bookings over longer ones on principal, and turned down extensions because I have another booking lined up later to have the second booking not turn up or cancel.
No booking is definite until the client has turned up and is leaving happy. Even mostly reliable clients can have something come up. That said you can sometimes tell if someone isn't going to confirm/turn up etc. Sometimes, very rarely I have double booked and 9 times out of ten been right. The 1 time I didn't let the person travel, I cancelled and we did rearrange for a slightly different time.
Yes reputation is key however I have been slagged off on the phone because I can't see someone, had regular clients unhappy because I'm fully booked by those who then drop out at the last minute - and last minute notice is good for them. I've also had new clients offer me extra money, make wonderful promises to try and persuade me to drop other plans or appointments to see them. The thing is I have no guarantee they'll turn up, sure I could cancel the shorter appointment and the other one might not turn up.
Regulars will stay with you but if you are unavailable enough they tend to make their own provision.
Following spates of having lots of new to me clients dropping out, I have on occasion reverted to regulars only for a while. Makes more sense to keep time available for them, and earn a bit less for while. Speaking of which at such times I've had the odd guy ask me to make an exception, because after all he had been in touch before I'd decided to stop seeing new clients - for him then to do exactly the same thing.
There is no answer apart from apply sensible rules and stick to them. Genuine clients should not suffer however what genuine clients do not know is the number of men who book, tell you they are genuine and will be there "110%". As if someone who's going to be flaky wouldn't say that?
What is interesting is the amount of cancellations. I'd estimate at least 50% of new bookings do not follow through either not confirming or let you know by cancelling, second appointment 75% turn up, after that around 90% of regular appointments attend. Some regulars never cancel, ever. One or two cancel every other time. If number of cancels = more than actual appointments attended I no longer hold time.