I know of some museums, particularly the Nordic Heritage Museum in
Seattle, WA that have a form for "drop-off" donations (when the
prospective donor drops off some object(s)  for the museum to consider for
acquistion).  The donor indicates on the form whether they would like the
item back if it is not accessioned OR if the museum may dispose of it (or
any portion of the collection) as the museum sees fit.  This also clues
the prospective donor in to the idea that the museum may not want/need the
objects.

Hope this helps,
Paula Johnson
Museology Grad student
U of Washington, Seattle