Statement mailed with finance charges attached. Phone calls to accounts payable to ask where payment is. Phone calls to your contact making sure everything was delivered and acceptable - you're just trying to determine why payment has been delayed.
Personally, the phone call to accounts payable usually works. If not, the phone call to your client contact usually gets it resolved.
Refusal to do more work until past invoices are paid or mentioning that you've got to postpone development on their project for other paying clients (this can backfire).
We also have an automated system that sends out reminders - our response is usually, yes that is an automated process, but, while you're on the phone....
