Don't go looking for affiliate products to promote UNTIL you have identified a market.
Now, you CAN go looking at what affiliated products are out there in order to IDENTIFY if there's a market or not, but the mistake most people make is building the affiliate campaigns around a specific product.
Most newbie guides tell you this, to do PPC and SEO on the product title etc. And this CAN be good - esp. if you're in a niche that likes reviews of specific products.
BUT! For the most part, what you want to do is market to the NICHE - the PEOPLE - identify a group with a common desire/passion, find where they congregate and where/how they get their fix.
Then, GET IN THEIR WAY. Get some content put together that interests them. Show them that YOU are a better source for their fix than wherever forum or blogs they hang out at now.
NOTE: THis isn't hard to do - you don't even need to be creating new stuff, almost no small niches really have news aggregator sites - with a little RSS and list subscribing yourself, you can become the expert source of info simply by spitting out everything that comes your way.
Anyway, back to the point - once you have the ATTENTION of the niche market - the PEOPLE - THEN you start throwing in your offers - affiliate products, special reports you charge for, JV offers. Sell advertising, hell whatever.
Focus on the MARKET, not the PRODUCTS. Get the BROAD interest, and you can sell them a TON more, a lot more easily.
Now, once you HAVE the attention of the market, THAT's when you can start doing the REALLY killer product name targeted campaigns, because by that time, you have the trust of the people in that niche. You're already going to have the backlinks from authority sites, and you can rank for your niche terms in the SERPS pretty much effortlessly.
Hope this little bit of "work smarter not harder" info helps out some of the people that are struggling.