We use the following letter and adjust it as needed.  We DO work with a
subscription agent, but have found that there are many small locally
published newsletters and such that a subscription agent won't handle for
us.

Dear Sir or Madam:

The [NAME OF OUR LIBRARY] has maintained a subscription to [TITLE] for [MANY
OR SEVERAL] years.  We would like to have a complete collection of this
publication for research and archival purposes.

We received the [DATE] issue today, however we have not received the
issue(s)for [DATE(S). Would you kindly send them to us so our collection
will be complete.  [FOR NEWSLETTERS WE ADD: IF ORIGINALS ARE NOT AVAILABLE,
WE WILL GLADLY ACCEPT A PHOTO COPY]

Please send all materials to us at the following address:

        [NAME AND ADDRESS OF OUR LIBRARY]

Thank you in advance for your assistance.

Hope this helps.  We find this format is flexible for the differing needs
with each title.

Beverly Butler
California State Library
Acquisitions / Serials
bbutler@library.ca.gov
916-654-0101