A horse's head in their bed works quite well.

Seriously though, people aren't paying because they're not paying -- not because they somehow forgot. They're assholes. So treat them like assholes -- stop providing services the second their payment is late. Demand payment up front. Do not fall for their gaslighting. Feel free to use the word "gaslighting" with them.

In some cases you just have to treat non payment as part of the cost of doing business. E.g. if you are performing services for a startup, assume they will flame out at some point and your last couple months invoices will never be paid. Price accordingly.

IANAL but also if you're in the US note that when a customer enters bankruptcy any payments made to you during the last 90 days prior to their filing may be "clawed back" if judged to be paid unfairly vs other creditors. So even if you did get paid by a crashing startup, you might have to pay that money back.

Edit: you asked about workflow. This is what we do: the second an invoice is overdue, send an email to their AP people, copied to the business contact (whoever you deal with in providing services) noting that the payment is overdue and asking when they expect to remit payment. How terse you make this depends on history. If they reply with some nonsense about check runs and things being sent out on a Thursday, respond with a message clarifying when you expect payment based on what they said. If no reply within a day or two escalate -- send an email to the business contact requesting a zoom call to "discuss their payment situation". So far the process has been in service of showing them that you take payment on time seriously and are going to grind their gears until you get paid. Often this will work. If not then escalate further. Whatever lever you have to pull, threaten pulling it (stop service, they don't own the copyright to code you wrote for them unless paid, etc). If you have no lever, well that's bad.