Ian, I read a book written by an accountant. In it he described various ways of improving things for your customers that ultimately makes things better for you.
One thing I remember was to make it easy for your customers to pay you. He suggested prepaid envelopes which worked for him. Now I know you are always telling people not to pay for postage, but it seems to work for some.
I will find the book later and post the title. I found some good ideas in it.