You sell goods; you issue invoices for those sales and hope for your clients to pay up what they owe to you. If they pay their debt in due time and in full amounts, the payment discipline is generally considered as good. However, once they begin to have overdue balances and problems with meeting the amounts, the payment discipline has changed and for worse. 

So how would you motivate your clients to make them change their payment discipline for better once again? Surely you don’t want to accept such behaviour so you must implement some measures.

Measures normally used are prepayments or not sending out any goods once all previous debt is paid up. Obviously there’s the relationship, the environment you operate in etc. You must also consider how forceful you can get with your clients without losing them (consider also if you can afford to lose them or you’d be better of losing them for an example).

Keep in mind that you sell goods your clients’ need, if they are happy with your service and prefer you to your competitors; you can and should enforce some means of measure for discipline in case any bad payment behaviours. It’s not good business practice to not pay to your suppliers.