Kami,
We do have a gift policy that covers all donated material, but the issue of
upsetting would-be donors is still tricky.  I usually use some combination
of the following, depending on the circumstances:

1.  If I know I do not need it (for example, National Geographic), I
explain that we have all issues bound, and I thank the person very much
for thinking of us.

2.  If I think I might need loose issues to fill in missing items, I ask
the person what years they have, and ask them if I could check to see if
we need any, again thanking them very much for thinking of us.

3.  If it is moldy, old, not relevant for us, or otherwise useless to us,
I explain as diplomatically as possible that it is not a candidate for our
collection.  I tell people I do not add old materials, only current
subscriptions (which is true), and I explain the realities of shelf space
in a library.  And then I thank them very much for thinking of us!

Nancy Williamson
Nassau Community College Library
<WILLIAMN@LIB.ACS.SUNYNASSAU.EDU>