I think what the SEO BOOK is one of the most ‘must to read and pay attention’ sites which ever know. In that article author is discover an interesting 10 tips how to and what to before choosing the niche.

And powerful tip which I saw is to join into the which ones you know something or want to know about, and small little very important point have a commercial value.

SEO & PPC: Finding Lucrative Niches: http://www.seobook.com/niche-hunting