As soon as you discuss taking them on set out your stall:-
Generally -
Them: "How much do you charge?"
You: "It depends - I offer monthly and two-monthly rates and the first clean is always rather more as they need bringing up to scratch."
Them: "Can you ring me before you come so I can be there/unlock the gate?"
You: "That's not necessary I have a "special" gate ladder with rubber contact points that lets me gain access with no damage and as I don't work in heavy rain or snow I cannot give an exact date each time - and if you're not in I just leave a slip for you to send me a cheque or you can even do it on-line now."
Them "It really is impossible for you to get round the back" (fort knox jobby)
You: I can do the fronts every month and you phone me and I'll do the backs as a one-off early in the morning/late in the day" etc.
or
You - producing a set of keys - "lots of my custies trust me with a key or code, would that be a way forward?"
If they won't play ball then do you really want them? This filters out most of the crud but have some well rehearsed polite but firm phrases:-
Them: Can you leave it this time?
You: Oh why? Is there an emergency/problem?
If it's an out of character real emergency or they've forgotten to tell you that their having work done then I would put up with it once in a while but if it's "I don't think they need doing this time, or come back in spring" Then:-
You: Well, I hope you understand but don't operate a "turn up and see service" but I can offer you a regular clean every two months if you prefer.
I'm brilliant on a keyboard mind but sometimes I just dump.