Find out who has the authorization to pay you and get to them as quickly as possible. In some companies, the decision maker may be an accountant, while in others, it's the owner. Don't afford to allow accounts to fall past due because you failed to find the cracks in the system. Make your presentation based on what you've learned. Collectors should use the information they've gathered to place the customer into one of several types and make their presentation based on that.
Customers are different, and the reasons for their delinquencies are, also.
Maybe a customer doesn't think you're important enough to pay promptly. Maybe he's not satisfied with the product. Put that information to use and customize an approach. Determine who is supposed to do what and when. If you call too soon, you will irritate the customer.