"Ultraniching" is the art of finding demographic niches within a larger niche. Example: Webmasters > adult webmasters > niche webmasters or Doctors > specialists > subspecialists. Normally, the deeper you go in the niche, the less the traffic but the higher the payout since many advertisers are looking for highly specific niches. We've explored this issue at
http://www.highrevenue.com/business-...-online-forums
Some key tips on how to mash search engine keywords, niche marketing, and "population" techniques to get a niche forum going.
One possible approach--start with the sponsor first. Find out what they are looking for and building around it.
Of course, this is tempered with the target market's actual online behavior/social patterns.