May of the clients with these types of issues do not pay their bills in general.  If you are going to accept a payment plan it is ideal to have payments stay ahead of billings.  If your client is unable to stay in front of the work with payments, you should at least have a set payment plan and payments should be processed automatically (i.e. ACH payments).  If you rely on your client to be able to pay, you will find they fall behind almost every time.  And once again,  If you resolve your client’s problem, you drop to the bottom of their payables list like a lead weight.