Acknowledge the loss and affirm that you are still eager to get their business. Let them know that you're still around on a regular basis and are always willing to help them.
1234, Main Street
Boston, MA 02123
08 April, 2005
<Recipient Address Goes Here>
I understand that you decided to choose another company <lost opportunity>. Naturally I'm sorry that we didn't get your business, but I hope you will consider us in the future.
Although we were unable to meet your criteria this time, please keep in mind that Abc Company is constantly striving to serve companies like yours. We are very proud of our quality and the value we provide our customers. If, at any time, you feel that you are not getting the service or quality you deserve from your new supplier, please give us a call.
In the meantime, I will contact you from time to time to keep you posted on our newest <what you provide>, and to see if there is any way we might be able to help you. Your business means a lot to us.
encl: <List of enclosed items goes here>