When I worked for HMQ I made a point of not joining the IPMS (Institution of Professionals, Managers and Specialists - I was officially a specialist), who I felt were a complete waste of space.
Now I'm in the civil sector and the Chief Engineer it seems a bit pointless - I can just threaten to resign and leave the CAA approvals suspended. However, I can recommend the RAeS, they may not help out in rows with your employer, but is the only place to get a good cheap lunch in London.
[This message has been edited by Genghis the Engineer (edited 03 October 1999).]