I always take my time with new custies. Same as Ian, I talk slowly...ish and explain I leave the windows wet.
The conversation goes something like...
I use one of those pole things to clean your windows, have you seen one before? (I never get technical).
No.
They are really good and the best bit is I don't have to use ladders so they are much safer.
Oh
It's basically a pole with a brush on the end and the brush is able to get into all the little nooks and crannies much better than my fingers ever did and gives the frames and sills a real good clean.
OK
Now this is going to sound really weird, but when I've finished I leave you windows soaking wet. (I over emphasise it as though even i'm surprised ).
Really?
Yes, but when they dry, they dry crystal clean. It's really clever stuff.
Also by the time i've finished, some of your windows will be starting to dry so I'll show you how well it works.
Never had much of a problem from then on. I think part of the trick is to spend 2 mins just talking to them and get them to like you.
Another trick I always do (especially on first cleans) is when i've finished, go round the ground floor and wipe the underside of the small openers (so they don't drip on the window below). Also wipe underside of the windows just above the window sills and the sills themselves. Get them fairly dry. This gives the 'apprearance' that they are not 'dripping' wet and the new custy will always look at the sills (which are now perfectly clean and dry).
Also there is usually one window that is starting to dry (even if it's a small area). Show the custy that although the window hasn't completely dried, this area that IS DRY is now completely clean with no marks. The rest should dry like that.
Sounds very long winded, but in practice adds another 2 mins to the clean with a very happy custy.