Hi VGC
I think that most window cleaners have come across this problem,
1, Try to collect the money when you clean the windows,
2 ,Try to sort out a place they could leave the money, under a plant pot or something.
3, Leave a sae and get them to send a cheque ,
4 Tell them you are now only cleaning the windows of the houses that can pay you when you have finished
5 Every new customer you get tell them you will clean the windows if you get paid as soon as you have finished, then get rid of a call back
Hopefully one of these could work for you