A good way to get in the door with existing customers is to have something new to tell them. Emphasize how you value their business and close the letter with a call to action.
1234, Main Street
Boston, MA 02123
08 April, 2005
<Recipient Address Goes Here>
We are pleased that you selected <name of product or service> in the past. Now we're excited to tell you about something new. Beginning <first available date>, <new product or service>
If you would like <next step>, please call us at <number>. We will be happy to <offer>. Thank you for your business and we look forward to working with you in the future.
encl: <List of enclosed items goes here>