The Institute of Fundraising's Special Interest Group is the representative body for prospect researchers in the UK. They hold an annual conference in London once a year and several smaller courses all of which provide invaluable training, advice and support for those carrying out prospect research. A list of past topics for each conference can be found on their website.
The Association of Professional Researchers for Advancement is "the premier international organization for fundraisers who specialize in research and information management." Based in America (it was the American Prospect Research Association before 1994) it offers its members a variety of research resources including a Professional Development Center, an Annotated Bibliography Center, Peer Advisory Guide and Connections, a journal of scholarly articles about prospect research, as well as various symposia and an annual international conference.
Brian Dowling's impressive site claims to be aimed primarily at those in higher education, but the section on research resources is relevant to all researchers in the non-profit sector. A highly valuable resource.
Not to be confused with Researchers in Fundraising, PRUK is the leading independent forum for UK-based prospect researchers. Keep abreast of the latest research developments, news, jobs, etc. or search the archives for past research wisdom (whatever problems you face, the chances are someone else has faced them before and asked the discussion group for help).