Once you narrow down you list of "potential niches" you need to identify keywords and research the competition. Competition is fine but you need to determine whether you can carve out your own space in the niche - have a unique position to attract a following, etc.
It helps, too, if you can build a picture in your mind of WHO your target customer is. Never forget the 'niche' you are going to target is composed of real people. Solve their problems or fill their needs - and you have a profitable niche.
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References: http://www.warriorforum.com/main-internet-marketing-discussion-forum/1288900-looking-reliable-niche.html?

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