I always chat to the client a bit at the start of the booking and ask him what he's up to for the rest of the day. I then slip in that I have a dentist appointment / tradesmen coming over to fix something / landlord popping over/ family visiting/ a friend's birthday party/ vet appointment/ hair appointment etc.

If the time is up and the client is still lolling about on the bed 5 minutes later then I make a point of glancing at the clock and going "shit is that the time, I've got to go to...." (whichever excuse I have previously said).

I always make a joke about how an hour flies when in good company. Or I say something like "thank god I checked the time as I was having such a nice time and could have sat here shagging/chatting for hours".

My regulars  never stay over their allocated time as I always tell them my funny "horror stories" of annoying clients who stay over their time or push my boundaries. They always get the hint that they'd better not step over line 

Non verbal communication combined with verbal communication

1. Pick up clock
2. Hold out hand
3. Say: "Give me more money" 

Those ones usually come back.  Weird eh?