Don't give the customer a chance to dictate to you. When asked how much you charge, have a look and give your quote like this, "I charge £X a month, but because your property hasn't been cleaned in a while, the first time will be £XX."
I've been messed around a good few times when people ask 'not this month', and I never ever return. But you'll end up with a hard-core of regular monthly customers.
Be firm and just tell them that if they want their windows cleaned by you, then they get it done every month. No exceptions (or very few).
Saying that, I fancy saving myself a few quid. I think I'll phone BT and tell them I won't be using my phone next month and to not charge me the service charge!
Remember, some customers don't want a decent window cleaner. They want a MUG. Don't be one.