Act like you'll get paid

There are two keys to getting paid. First, invoice promptly. Second, act like you’ll get paid.

You aren’t asking for permission to get paid. You aren’t fitting in at their convenience. You aren’t giving them a 90 day interest free loan. You won’t be put further back in the queue than any other suppliers. You didn’t do all that work only for them to question it and make a generous  offer of 10%

You had a contract under law for a service or product and they have an obligation to pay. Act like they will pay on time and you’ll be surprised at how well it works.

If someone has a history of paying late then just ask for payment in part or whole upfront.