[NAME, COMPANY AND ADDRESS, ex.
John Smith
XYZ Inc.
1234 First Street
Suite 567
Anycity, Anystate  85245]
 

Dear [NAME, ex. John Smith], 

It has been a pleasure to do business with you during the past [DURATION, ex. six months].  We find your firm to provide fine services.

[STATE REASON(S), ex. As you may know, our firm has undergone several fundamental changes administratively, and in the way we do business.  We now have a dedicated marketing staff which will handle our advertising and marketing plans.]  We regret, therefore, that we no longer require your services. 

Please accept my thanks for the services you have rendered to us in the past.  We certainly wish you the best in the future.

Sincerely, 

[YOUR NAME, ex. Jill Jones]