An email newsletter is a great way to maintain contact with your customers. If you have information that will be useful to your clients, a regular newsletter both imparts the information and at the same time reminds them about how they can benefit from your business.
To avoid spamming, It is essential that your newsletter is only sent to customers who have requested such contact. Managing a mailing list with options to opt in or out can be a relatively simple matter. There are services available that will help you to build and maintain your mailing list. Many of them provide statistics on how many emails are received/returned, opened, clicked through etc.