Bringing in a collection agency where a mere phone call would have sufficed is not using your resources wisely. Call the debtor and fax over a copy of the outstanding invoice. Then send a letter, then send a registered letter. If the debtor is a business, submit a detailed and strictly factual complaint to the Better Business Bureau. If all these actions don't get the bill paid, it may be necessary to call in the hired guns.
Experts suggest two easy systems for deciding that the time has come.
In a time-based system, you might collect for the first 90 days. After that, a collection agency gets the account for a year, and if the agency can't collect, you hire a lawyer. On larger past-due accounts an agency typically will both send letters and make phone calls. On smaller bills the agency may only send letters. There is also a lot an agency cannot do.