How-to articles do well, counted-off articles do well (5 ways to lose 10 pounds, the top 3 best anti-virus software programs, 10 money saving tips, etc). "What NOT to do" articles also do well (Avoid this common mistake when starting a rose garden, What never to do when trying to meet new people, etc).
Make sure the article isn't more than about 400 - 500 words long. Make sure you break up the article into very short paragraphs, or the reader will just see a huge block of text and figure reading it will be too much work. No paragraphs more than 3 sentences really.
Your articles do not need to be precisely about the item you are trying to promote, but rather on the same subject that your item is a part of. Say you sell an ebook on rose gardening. No writing articles on why they should want your ebook! Just write articles about the kinds of problems rose gardeners have and how to solve them, and then at the bottom of the article in the author box, write something like, "for more helpful tips on Rose gardening like these, visit example.com" which is your site, where you sell your rose gardening ebook.