Late and unpaid customer invoices hurt your cash flow. Develop and put into place a process to regularly review the aging of your Accounts Receivable. Set specific points in time each month to contact customers and put these dates in your calendar with reminders. Follow up on unpaid invoices by sending out statements, duplicate invoice copies, and/or picking up the phone. Consider setting up payment plans with customers and instituting fees or interest with increased aging (e.g. after 30 days). When a customer payment is received, apply it against the corresponding open invoice to ensure that your Accounts Receivable balance is accurate.
Remember, every late payment is an interest-free loan to your customers that negatively impacts your cash flow.