How do you deal with non paying or late paying customers ?

Line is: Warning - Meeting - Legal actions - Sale of a debt

Well, By tracking the aged payments updates through the email notification for their information, That may help to allocate their budgets, and also it leads to get track of follow up. it helps us for review and corrective action. 

Keep everything contractual, clear and well-documented from the start. This way you can reduce the risk of not getting paid on time, and will put you in a more powerful position if payments are late.

Understanding whether is customer is ID (Intentional Defaulter) or IAP (Inabiity to Pay) is very important & foremost step in deciding what step to take to cover the debt. If he is ID, then one must be harsh on collections part & must adapt methods ranging from frequent follow up via CC (Call-Centers) to/till taking legal actions. In case of IAP, it is mostly because customer don't know how to manage his money. Thus, providing him solutions for money management as per due date is best solution we can provide.

Agreement condition very important at all works stage, so you must to control your completion with receiving payment.

In general, nowadays, a punctual and timed payment is very important in order to keep a company profitable and its operations ongoing.

A set of arrangements and negotiations beforehand is a good start. A description of the payment details and deadlines included within the contract are also important, in case legal actions need to be taken.

Before resorting there however, a kind reminder personally and/or electronically should take place. Common understanding in such issues is vital for a business relationship to be ongoing and fruitful. However, should these customers remain constantly unreliable, it is really up to you to evaluate whether to cease doing business with them or not.

I hope I was helpful. 

First find what is the real problem with them, if its regular customer then give them solutions, which will meet both company terms and condition,

If new customer then inform them that as per agreed payment terms , Late payment fee may apply

Send them gentle reminder

Send them kind reminders ahead of deadlines. Remind them as well with the terms and conditions agreed.

talk to them nicely , ask the customer the reason why they pay late and tell they need to pay their monthly dues or obligation ( debts ) or else they will be charge according to the agreement.