Whatever you need to promote online with affiliate marketing you really need a good niche, like a good pair of smelly boots a niche can be quite useful to attract people(smelly boots?)
Ideas for an affiliate niche to be selected could be the ones that are in large general niches such as health, business, entertainment, design, travel and then go deep into these huge niches and find some little known but quality affiliate programs that will build in terms of earning potential and value for your referrers and your own earnings.
Lets look at health affiliate programs, the popular ones seem to be health drinks, berry drinks for this and that, I see them everywhere, so to target that area with an affiliate promotion would take a lot of work to compete with all the other larger companies who have super affiliates already in place, instead finding a little known health drink or a new one would be ideal for your promotions to be sustainable in growth.
I myself have never ever promoted health products as yet, but there may come a time when I just might, so I'd probably go for a health product that is relatively new or has not been promoted to death by large companies or health and wellness corporations. And part of my marketing plan would include an adwords campaign too as that would drive the traffic that much more quicker to the promotions.
More and more professional affiliate marketers will tell you that you should be promoting an affiliate program based solely around your interests and that is what is important, to start with an area you are familiar with and work from there, also by creating value around sub topics of the main interest that you are really into, you can build up sub niche topics with valuable information that is useful for your customers.
Experience dictates a lot of the online decisions you make, because without any experience in a certain area then you might just create content that is poor in terms of not fully being researched properly.
Some other ideas for a niche affiliate promotion are:
Magazines - these are great because there are magazines for almost all kinds of topics for hobbies, health and fitness, business and many other niches, just take a look at your local book store or newstand for the specialist magazines, this will give you lots of ideas and actually pin pointing a magazine subscription to locate and promote.
Around your house - Eh??! you heard, everyone buys something that they like a book, a dvd, software, laptop, whatever the case, you can find most of your favourite products in an affiliate program, promotions work best when you own the product and have used it, therefore you can recommend it based on your own personal use, why it is good, why you bought it, write reviews all over the internet and provide this useful information. Internet customers actually read reviews to the product to gain more info before they buy, I know I do on Amazon.com.
Search online - Searching online for specific ideas does work best when you have a good idea on what you want to promote in the first place, searching the affiliate networks or affiliate directoriesis a good place to start, but also typing in a specific phrase and then affiliate program after it may come up with more direct results.