I try not to let moaners get me down and try to adopt the attitude of 'Well, if I lose 'em I lose 'em, so I may as well just get on with it'. I'm firm but friendly with them, and I can't think of many moaners that trouble me these days.
I've recently converted all my work to 100% WFP, and mostly they don't seem to care, but I've the odd one whom I suspect don't like it. It looks to easy to work properly doesn't it?
My current line is to give it three cleans, and if they're still not happy we'll discuss the situation; I've implied we will go back to cleaning the old way (WFP tops, squeegie ground floor); but in reality, if they're not happy, I'll ask them to find another window cleaner (and they'll struggle to get a trad cleaner around here; trad cleaners are a minority around Chepstow).
And for the other odd 'funny burger' we may have, well, as long as I do a good job, they can moan all they like; it's only words and only affects me if I let it.