Once your company’s efforts to collect prove fruitless, it may be time to turn to a collection agency. This will allow you to nail down a partial write-off. When you hand over uncollected invoices to an agency, you agree to pay a certain amount to the agency. You can treat the fee as fixing a partial write-off of your receivable immediately.
One of my clients has an invoice that is five months past due. Time to bring out the baseball bat. I know a few good lawyers.