You can take advantage of the WordPress.com Related Posts functionality to return related posts quickly. Enable it by turning on Related Posts in Settings > Reading and you’re done!
If you’d like to programmatically retrieve and display posts within your theme code, you can do so like this:
$posts = Jetpack_RelatedPosts::init_raw()->get_for_post_id( $post_id, $args ); // Returns an associative array in the form of id => $post_id: /* array( array ('id' => 12345), array ('id' => 54321), ) */
Related Posts indexing will work regardless of whether your site is marked as “Private.” For self-hosted sites, Related Posts will currently only work for standard post-types; we are working on enabling it for custom post types.
For more information about ways to customize related posts, see http://jetpack.me/support/customize-related-posts/