if you want to get psychological minipulation
try this trick I once saw on Tinmen ( a film with Danny DeVito & Richard Dryfuss)
ask the customer if they are happy with your workmanship, when they yes ask them if they will recommend you to thier friends. Most people will say yes, then tell them that you give a £5 referal fee to all customers who recommend your cleaning services.
then say " because I can see you are an
honest & trustworthy person and you've said you will recommend me to your friends I'll give you the first £5 referal fee right now!" and then hand them a crisp, clean £5 note.
I'm sure you can see the psychological implications of what you've just done
Mike