When a client ignores the payment terms on the invoice you sent and takes a long time to pay up, you can send a polite reminder to the client in case he simply forgot to pay. If you receive no response after sending the reminder, consider giving the client a call – phone calls are harder to ignore.
The next step is to hire a lawyer to write a formal demand letter and send it to the client to let him know that you’re serious about collecting the payment. When all else fails, it’s time to take the client to court. When you do this, you’ll likely lose this client forever, so you have to make sure that you can afford the risk.
Alternatively, you may consider offering a discount to clients who pay early as a strategy to reduce late payments.