The article directories let you place a signature or author resource box at the end of each article you submit where you can put a live link to a website or specific product you are promoting. The search engines will pick up the links and help the website or product you're promoting to increase its Google page rank, as well as drive your traffic directly to the site- a win, win situation for all.
One note: you will need to purchase some top-level domain names to redirect to your affiliates, as most article directories do not allow affiliate links in the resource boxes or articles. You can purchase.info domain names for $1 per year, so they are the most cost effective.
Pick a niche--better yet, pick several niches and diversify. Even though a product looks like a winner, you won't know that until you promote it and see what kind of action it gets.
Then pick a product for each niche you've selected. These products will be what you promote, and you will link your articles about related things to. You can find products to promote by joining some affiliate websites, unless you already have a product of your own--then by all means, promote that! Clickbank is the biggest affiliate website, but there is lots of competition there, so look into others like Commission Junction also.
Pick the best keywords to promote your product and be sure to sprinkle them through your articles.
Next, research your topics and write your articles. Make sure that some of your keywords are in the title, once or twice in the first paragraph, and the last paragraph. Write good, compelling articles that people will read from start to finish- often just making an article "10 tips on…." or something similar will get them to the end. You want them to be interested in what you've written, and want to find more. (This is where your links come in….)
Write many, many short, good articles--they don't have to perfect, and the ideal length is 300-400 words. The more articles you write, the more successful this method will be. The true key to success is how effective your signature, or bio box is. Write this carefully, because it is where you link to your site, or promote your chosen product. On those article directories where you cannot use affiliate links, you'll need a presell page instead, like a blog.
Post your articles to directories that have higher page ranks - you want to only post on PR 3 or higher pages. This gives it a head start in popularity, as articles theoretically inherit a portion of the search engine ranking from the site where it's posted.
If you've followed these steps well, you should begin seeing some money coming in from the affiliate products you have promoted.
Obviously, if you write 50 articles for four different products, and one of them outperforms the others, you'll know how to tweak your efforts for greater profit. The good thing about this method is that your articles will be read, and people will buy the products long after you've written the articles. I know of one person who wrote 500 articles over five years ago who still receives about $5,000 in annual income from those articles to this day.