Many blog posts eventually disappear from search rankings because they ”fade from memory”. They are no longer on the home page. They aren't linked to often. They just fade away. You can't expect every blog post to remain in the forefront of search engines robot-minds, but you can help keep a select few from doing a disappearing act. The more often you link to your own posts from within other posts, or from your sidebar, the more chances those posts have of being kept in the search engines main index. Some of these links will be manual because YOU decide certain posts are important, and some can be auto-linked using plugins.
Jaw Popular Posts WordPress Plugin
Yet Another Related Posts Plugin