Managing payment plans AFTER invoicing

When we have a customer that cannot pay their invoices we will sometimes setup a payment plan with them.  The terms are often quite different, for example we have one customer that pays $10 a month for almost 2 years!!

Here's how the process goes for us:
[LIST=1]
[*]We enter a sales order
[*]We ship the goods and invoice the customer
[*]Once they are past due we begin contacting them
[*]If they need a payment plan we setup Outlook reminders to send statements and or invoices at the agreed upon interval[/LIST]
This has worked fine when we didn't have very many of these situations (5-10 a month) but we are seeing an increase in the request for payment plans and I wanted to setup something more robust and managed, something that didn't rely on basic reminders.