I tell donors that having an item on permanent display
is not good for the artifact.  Exhibition gallerys reflect
visitor comfort levels and while not harmful to an object
during a short term of exhibition, the slight increase
in humidity, light, and dust can effect an artifact over
a period of time.  Putting an artifact back into storage
allows it to rest (I've never had to explain "rest" to
a donor.  They just seem to accept the term).

 I also add that local visitors become
indifferent to an artifact that has been displayed
forever and no longer recognize or care about the
importance of the item.  By putting it back into storage
and exhibiting it again later, the public has renewed
interest.

When possible (as with easily moved artifacts) I
assure the donor that I will bring the object out of
storage for any member of the donor's family to see.
This works well for people who want to show-off for
out of state relatives who are coming to town.  I require
two weeks notice so I or a member of my staff can be
there with the artifact.  I tell the donor this is so questions
concerning conservation and exhibition can be answered.
Actually it so I can be sure to find the object (especially in
my last museum where proper storage and object location
were battles I was losing with the board).

There is no use in bringing up that there is not enough
gallery space to properly exhibit every artifact.  Donors
only care about their donation and not your problems.

Hope this helps.

Belinda Nickles