If my Customer is out (and most are at work) I leave a stamped addressed envelope and an invoice. This reads to the effect of...'your windows have been cleaned today'...........include name, address,date, and a little message on the bottom saying 'your prompt payment is most appreciated', dont forget the price...and importantly, use a first class stamp ! Never devalue your business or the importance of payment by using a second class stamp ( all it sais is pay me when you get round to it ) I use a PO Box...I call every day to collect mail and pay the cheques into the bank. I draw it out on Friday....simple. Anybody doesn't pay by the next time they are due...doesn't get done again until they have paid. Tunes the round, trains them up !