I get paid a few monthly standing orders for jobs I clean monthly. If I'm ever late, ie receive two payments without a clean, I refund a payment through their letter box in cash when I tip up to clean.
It requires the customer to trust you.
For the rest, and I have a couple of houses that are let and the customers don't live in the area (one in London the other in New Zealand), I just e-mail them and they pay me by bank transfer.
The others I just leave a note and they pay me by bank transfer.
I advertise this on my 'Your Window Cleaner Has Been' chits and quite a few pay this way now. I get paid quicker from the bloke who lives in New Zealand than Mrs Smith who lives down my street.