I have a friend who retired but still does some contract work. They were on salary their whole career and are not used to sending invoices and tracking down payments. One of their clients was late paying and my friend wasn't concerned, but I encouraged them to be diligent about insisting on being paid on time. I have been a freelancer for over a decade and in my experience, the further away you get from a bill, the less real it becomes to the person who owes it. They start forgetting what the project was, or worse, start questioning why they even have to pay for it. You have to stay on top of these things or they can spiral out of control.
Just for my own general curiosity, once a bill is overdue, how often do you nag the customer about it? Or does it look more like an escalation process? (Start out with a polite email, then a phone call, then a phone call to their manager/business partner, etc?) Do you ever "fire" customers who always pay, but always pay late and only after they've been reminded 3 times?
They get 30 days to pay. I used to start sending reminders if they were late by even a day past that, but I quickly learned that can unnecessarily embarrass or annoy some clients, so now everyone gets a two week grace period after the 30 days are up.
I send a gentle reminder just to my immediate point of contact to start. If that doesn't work, I start sending more business-like requests for payment every week, and I CC anyone I think might help make it happen.
I've never fired anyone, and everyone has eventually paid (knock wood), but there have been times when I assumed the money was a lost cause (though never five figures worth like the person who shared this blog post).
But more than once, I've had a client contact me and request new work, and I've had to remind them they still have an unpaid invoice. I'll tell them I can't start anything new until the old work is paid. They almost invariably blame the delay on their billing department, and the money eventually finds its way to me.
Net 30 is a standard way to do this. Lots of examples online on how to add this to your invoices.
Well run businesses will pay you on the 29th day. :)