Pautang is part of doing business. Sometimes you have to do it to keep your clients.

1st - get the agreement into writing at the time of purchase, e.g. amount to be paid, date due.
2nd - on date due and payment is not received, send a notice. Do this up to three times, say every 30 days, depending on your cash flow.
3rd - send a letter indicating that the amount is still unpaid and you would appreciate immediate payment.
4th - have a lawyer/collection agency do it.

One other tip is to price your product or service in such a way that paying immediately has discounts.