Regularly update your debtors. Develop a system for recording all your telephone conversations and written communications: this will be worth your time. If you have many account customers, develop or purchase a purpose built database application.
Make sure your invoices are designed for ease of understanding, clarity of information, and speed of payment. Pre-emp any questions the payer may raise by including ALL information; and state the exact date for payment, not just '30 days'.
Take the simple (but important) steps of sending your invoices to the right person at the right address for payment. And send it on time. Taking the time to clarify this can save a lot of time later on chasing an outstanding account.