Candi,
  You have a valid question, as I have pondered it myself off and on. I guess I am so frustrated. Some of the clients I am attempting to charge a late fee are clients that pay all around the old invoices. Maybe they will pay a September invoice but leave some August and July invoices unpaid, but never explain to me why.
   I try to send them a reminder immediately after I get their check that they still have old invoices to pay. I don't think it sinks in with them as they often ignore the second statement. I have found that charging a late fee, even if they don't pay the finance charge seems to get us paid faster. Several clients paid those old invoices pretty quick this month. Then I usually delete the FC from their bill. So maybe it just gets their attention. But whatever works.
  It's unfortunate that we have to work so darn hard to get paid, but until someone comes up with a magical technique I guess that is the way it is going to be. 
  We really do have to jump through some hoops just to get what's coming to us, but there are some rewards or we wouldn't still be here.

Thanks for your feedback,
Sharon