Our policy is, at the end of 30 days and we have not received payment, we call the client.  If we don't receive payment when they say we should receive it and we have received another job, we call the client to let them know their job is being held until payment is received on the outstanding invoice.  We provide 24-48 hour turn around time, many times it is less than 12 hours turn around time, and our searches include a certain number of copies, so our client is getting a really good deal.  All we expect is being paid in a timely manor.  We have had some clients overnight checks to us in order to get their work returned that we have been holding.  This works for most of our clients that are slow pay.  If they are continually having to be called, we drop them from our list of clients.  Fortunately for us, this doesn't happen very often.  We do have some that refuse to pay at all, and we send their work back to them marked "REFUSED".