Agree, you have to keep on top of it.
Issue your invoice promptly on completion (having said you would. Preferably take it to the cust and put it in his hand, wait while he gets his cheque book. If he doesn't ask him to (many businesses don't issue bills promptly
which doesn't help them to get paid.
Go round at the end of the seven days and say, pleasantly, you've come for your cheque. Don't say anything else. Stand there (the silence may be awkward, but don't weaken yourself by being the one to break it).
Collecting money may be awkward and uncomfortable and you may not like doing it... but hopefully you will get used to it. It's the people who put it aside, as a job they don't like doing, who don't get paid. It's hard, and embarrassing, like when you were young and trying to ask grils out.
If the punter makes a promise to pay at the end of the week, take out your notebook, in front of him, and speak out loud as you write "Mr Scroggins...will pay...the £1739.50p owing...on Friday...14th December...2007" look at him and say "That right?" And pause. Stand there until he says Yes.
Come that due date, you must be on his doorstep (unless the cheque arrived in the post already).
If he breaks his promise, say sadly that you'll now have to go to the small claims court or whatever.
You may prefer to get someone else to do the chasing on your behalf. It can be your wife, business partner, dad, or a retired person looking for some part time work. He can always introduce himself as "the accounts department of Bloggin's plumbing." You can pay then a small commission if you like, even 0.5% is much less than your overdraft charges for a month, and a reasonable amount for you to pay.
A funny story (well it made me laugh) I once went to the office of a customer who hadn't responded to the usual phone calls and letters... it wasn't there. I don't mean they'd moved out... I mean their premises had been demolished. They hadn't paid the rent either and the landlord had repossessed and was going to redevelop it. I later found the equipment in a bailiff's warehouse awaiting auction. A contract showing title was still vested in my company meant they weren't entitled to seize it.
On another occasion, I tucked an order into my pending tray until the cust phoned me (intending to complain delivery was late) and when they introduced themselves, I said, ah yes, Mr Scroggins, you must be phoning to say why you haven't paid for the equipment delivered on...