Getting paid.
A FIRST REMINDER
"Invoice [number] for [amount] was due [date]. This customer is usually reliable. Write a short businesslike reminder. Under eighty words. Not apologetic, not aggressive."
A SECOND REMINDER
"Invoice [number] for [amount] is now [days] overdue and a reminder was sent [when]. Write a firmer follow-up stating the amount, the original due date, and asking when payment will be made."
A FINAL NOTICE
"Invoice [number] for [amount] is [days] overdue. Two reminders sent. Our terms say [what happens next]. Write a final notice stating the position and what will happen if it is not paid by [date]."
A REMINDER BEFORE THE DUE DATE
"Invoice [number] for [amount] is due on [date]. Write a brief courteous reminder."
That last one is the most effective and the least used. Most late payment is oversight.
FOR A CUSTOMER IN DIFFICULTY
"A customer has said they cannot pay [amount] immediately. Write a reply that acknowledges it and proposes [arrangement]."
WHAT TO ALWAYS CHECK
The amount, the invoice number and the date.