The problem with narrow niches is that is hard to get visitors to that particular niche without a lot of focussed marketing which is hard without spam email, which in turn is not practical at all. The idea is, pick a broad niche, that millions of people are interested in, and affiliate market that.. this strategy works out great to generate profit, and can even make more money than adsense. Because each click is usually a sale, and puts real dollars in your pocket instead of the beggarly 0.01 cents that google throws at us and we wag our tails and say woof woof.. Here is one of my favorites: www.horoscopes.ws This is a horoscope site. The affiliate button is hidden at the bottom of the page, that is where you can sign up or signin. Hope this helps.
I would also have to say that Parenting is another great niche to be involved with. There are hundreds of thousands of mom/dad bloggers that you can work with by building links, guest blog posting, running contests, etc. This makes it easier to get your foot in the door and always remember to build value!