When you have a slow paying customer that keeps telling you that the cheque will be mailed out tomorrow (next week, whenever) get a confirmed day from them and once they have committed (a lie or not) let them know that you are working in the area and will come down and pick it up. If this customer is out of the area or out of town, ask them what time you should arrange the courier for
This is typically a no fail tactic once they have told you they will be making a payment. I used it just yesterday! My cheque should be arriving via Purolator in about 2 hours.
Whenever possible deal directly with A/P, the guy arranging the deal originally usually has nothing to do with billing. This way you will also preserve your relationship with them.