I lost four customers straight off purely because they didn't even like the idea of it. Another one had seen someone working with the equipement and just thought it was terrible that the windows were left wet. She just assumed it was tap water and that the windows had to be dirty when they dried and she wanted to cancel. I offered not to accept payment if she wasn't happy after the first clean and she agreed. Upshot was, she paid, and now sings its praises regularly, even after five years.
One thing I've learned to watch for in the warm weather is open windows downstairs, and shoes that people often leave just outside doors in the garden. Customers who don't mind having their windows cleaned with WFP don't like water getting inside their homes or soaking shoes left in the garden. Also, many inward opening doors leak even when new. The little drain holes in the bottom of the frames can't cope with volume of water produced in short bursts by WFP. So if the door doestn't have a rain guard attached that stears the water over the frame and outside or if it's present but not in good order then I do them by hand.
When converting my first floor flat custies over I visited those on the groud floor beneath and who cleaned their own windows, explaining the changeover I was making and that I would leave a note in there flats on the day that I cleaned their upstair nieghbors. This would enable them to know my schedule enabling them to clean their windows without me comming along and messing them up straight away. To my supprise, some that wouldn't take me on as a traditional w/cleaner asked me to clean them with WFP. (So, More work!)
So far, in five and a half years of using WFP I only have one downstairs householder who is being difficult. She has only recently moved in and cleans her own windows. The previous chap was a window cleaner himself but never cleaned his own windows. (They were filthy) However, she isn't happy with my suggestion of leaving a card. She gives me the impression that she just want's to be awkward. But I'm still working on this one.