When that happens, you should move the discussion to the most appropriate 
newsgroup and maybe even change the subject line as well. There are several 
ways to execute this transition :

1) Do one last response of this thread in the original newsgroup.
   Somewhere in the post, mention that you think the discussion should 
   be moved to a new newsgroup.
   Set the follow-up of the post to this new newsgroup.
   (i.e. in header, put, Followup-To:new_newsgroup)

2) If only a few people are involved in this particular thread, email to 
   inform the several individuals about the change of the forum for future 
   discussion.

Better yet, do both (1) and (2).

, if the thread is "dying", don't bother to followup on it to 
get it into another newsgroup. Your post will only keep the thread
alive a bit longer.