I will add my recollection of an article I read some years ago which
compared various environmental advocacy organizations in terms of the
fraction of each contributed dollar which went directly to programs, as
opposed to raising more dollars and to front office costs.  The Nature
Conservancy was at or near the top in this ranking.  Furthermore, they were
probably the first group to understand that the way you preserve species is
almost always to preserve habitat.  I too have been a long time supporter
and volunteer.  If I had a million to give, that's where it would go.
 
An interesting history of the early days of TNC can be found in:
 
        Morine, David E., 1990.  Good Dirt.  Globe Pequot Press, Chester,
Conn., 195 pp.