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. :)