Hi Lee
We do a mixture of both residential and commercial and have slightly different terms of payment for both with commercial our terms differ from residential.
When I take on a new residential account I will tell the customer that we will clean the windows on a set frequency I.E four, six eight weekly, three monthly etc and that we don't return to collect payment. (In eight years of trading I've never done this) So it's the responsibility of the customer to settle the invoice if their at home during the clean, or to post a cheque or do a bank transfer within 14 days.
If you find an early sign of bad payment, collect the money that you are owed, then take that customer off your work list and consider them bad business.
With any commercial account I will need a work order before anything starts and a signature on completion of work, if the customer exceeds my terms by a couple of days I'll send a statement of account, then a reminder, then I'll use my debt collection agency to take them to to the small claims court and take a county court judgment out on them. I've only had to do this a few times but the accounts were large and I'd had to shell out for access equipment at my cost of up to 1K, anyway with a works order and a signed completion note I was able to successfully take out a county court judgement against a couple of national property investment companies. they gave me the run around thinking I was a small business and would write off the debt, but was soon aware that their credit rating was about to go red and when you deal in property and need loans from the bank this isn't good. Both companies left it until far too late then contacted me on a personal level to try and square up. I refused their offer and collected through law what was mine including costs, and late payment interest etc.
I have more work than you could shake a stick at down here in Bristol and don't seem to ever attract the bad sort. If ever anyone ever feels nervous and needs a heads up on possible bad apples in this part of the UK then please feel free to PM me and I could save you some hassle.