Don't over think it guys,
Just reflect it on your pricing. You will always get messers, in any line of business not just window cleaning.
I have a lady whom i took on as a Bi Monthly Clean earlier on this year; she had her 1st clean at £55 and then two others at £38,
then she cancelled one clean for some reason i cant remember. ok no worries ill give you a new date for the next one, so did that, she cancelled that but asked to resume in 2 months time, no worries i said. 2 months later, i do the clean and give her a bill for £55 instead of the £38. Whats this, we agreed to £38 every two months! Yes i said we did, well why have you given me a bill for £55, well, you havent had them cleaned every two months have you? you kept cancelling.
She was ok with it, and could underdtand the logic, she then said, i better stick to the dates then hadnt i? Yep i said. ..see you in two months..by all means call me if you need to cancel...no no she said we wont.
lol i just dont get worked up anymore...life is too short.