How do I archive old sales order numbers?

Before purging closed sales orders or invoices, at the start of each new year I create another company with all the data of the live company. I call this company e.g. "archive 2005" with an extention of "05" if it contains all data including 2005. Then I would purge 2005 S/Os and invoices. This will allow me to open the "05" company and run reports etc.