Hi
SAP has provide standard tools to assist in collection which is collection management. we have just start using it in my company
by using it we might be able to follow up with due invoices and you are able to create priority formulas to highlight such invoices which become over due
you may refer to this document which explain the collection management in brief ..
SAP Collection Management (Introduction )
http://scn.sap.com/docs/DOC-58680
Regards
Mahmoud El Nday