You should make it clear throughout your dealings with the customer that you expect prompt payment. Draw their attention to the credit period when they first apply for credit. Make sure you invoice promptly and follow up with regular chasing if necessary.
You may want to consider including incentives to pay promptly as part of your terms and conditions. For example, you might offer a prompt payment discount or settlement rebate for good payers.
Credit control can be much more effective if you make phone calls when appropriate rather than just sending emails and letters. A polite and friendly attitude can make a significant difference.
It’s worth being aware that many customers only pay their suppliers at weekly (or even monthly) payment runs. You should check whether your customers have regular payment runs — and if so, try to ensure that invoices reach them beforehand.