1. Write for people. Who is going to read your content? The search engines or people? Who is going to buy from your affiliate link? Search engines or people?
This should really be a no-brainer, but too many people are being sucked into automated page builders that do nothing but spit out spam. People want real, sincere, thought-out, and informative content. They want to learn something. Automated tools cannot do that. (You can read more about my thoughts on automation on my blog post.)
People will be reading your articles and they will also be buying your products. Google won't.
2. Write articles for one keyword or keyword phrase. Ok now that you're writing for people write about what they are searching for. If your particular niche is golf, then you'd do some keyword research on what people are searching for. A broad term like golf would have thousands of focused keywords and keyword phrases to build articles around.
You can use tools like Wordtracker, or the free Search It!. I really like both of these tools and use one of them everyday to help with both clients and my own articles.
Once you've found a keyword term then write an article on it. You can write as many articles as you want on that term, but don't try to stuff multiple keywords or phrases into one article. This can be seen as spam and Google will ban your site.
3. Keep it short, but informative. The issue here is people don't spend a lot of time reading articles. They want the info fast so they can move on to the next one. But, that doesn't mean we skimp on the quality. Most articles being written now for content are short, informative, bullet style articles that deliver quality content in as little time possible.
That means to long introductions, and drawn out explanation. Short, crips, concise, but conveying the points.
4. Use links, but sparingly. Here's another issue Google has been taking with affiliate sites. The excessive use of links, without any quality information, will get your site banned. Now don't let this be a scary thing. It can be for people who are using automated tools, but for the ones who are taking time to build a successful business, this is actually a blessing.
It requires you to build a site rather than just put up a bunch of meaningless pages. Again, good quality content is the key, but within in that content place two to three strategically placed links to the merchant site.
Again, if you site is on golf and your article is about taking strokes off your swing, you could mention a quality driver and link to it. This way you're giving information and sending the reader to the merchant site.
5. Be unique. The Internet is full of copycats and people living off the imagination of other people. Set yourself apart within your niche and write unique articles.
What I mean is present new ideas, new ways of doing things, and don't shy away from being controversial. Develop your own voice that will shout above the masses. A profitable niche site is the one that people are talking about and are re-visiting frequently. They will only do this if you give offer them something new.
Yes, you can write an article, or three, or even a hundred. Yes, you need to in order to build a strong, thriving, long lasting, and most importantly of all... profitable... niche site.
P.S. Ken Evoy has written an awesome ebook that goes further in depth on writing to Pre-sell your visitors. It's a must read for all affiliates!