I can't wait for the day when I have too much work so I can tell them "sorry I have my regulars to look after".
I've only had this happen about five times, but when stuff like this occurs; I'm brutally honest.
I explain that when I took them on we agreed to a set frequency of clean and an amount. I tell 'em that I've stuck to my side of the agreement, however if they want a longer cleaning frequency, I'll stretch to every other month at a slightly higher price; and would charge (for example £13 for what was a £10 clean) and I tell them why (dirtier windows take longer to clean). I also explain that it's just not viable to clean houses on an ad hoc basis, this is my occupation, and I've a family and bills to cover.
If they don't agree, that's fair enough; no hard cheese as they say. I'd thank them for their custom and walk away.
But I would also drop any customers who messed me about; straight away; and if you're desperately short of work, get your finger out and get more. It's not difficult with a bit of thought as to what area and a bit of a 'spiel' to canvass more up; honest.