We have had good luck with Credit Services of Michigan.  Typically we have them send two letters on their letterhead that threaten collection proceedings first, if the client is going to pay that will prompt them to action.  That costs just a few dollars.  They have also recovered from several clients through the collection process, keeping about 30% commission if they collect.  They have worked very well for our needs.  Good luck! 

http://creditservicesofmichigan.com/