Send a statement immediately at the end of 30 days politely saying payment is now overdue and please send by return. No payment after 7 days, telephone and ask where it is. Keep following up by phone say every 7 days or so, otherwise consider cutting service, I will do this after 2 to 3 months of non payment, depending on the customer. Some larger commercial customers only do a cheque run once or twice a month so you might have to wait till the next one. But the key is to get your invoices out promptly and follow up.