Create a simple Related Posts loop based on the current post’s categories.
$post_cats = wp_get_post_categories($post->ID);
$this_post = array($post->ID);
$query_cats = array();
foreach ($post_cats as $cat) $query_cats[] = $cat;
global $post;
$related_posts = get_posts(array(
'category__in' => $query_cats,
'post__not_in' => $this_post,
'post_type' => 'post',
));
foreach ($related_posts as $post) : setup_postdata($post);
// Your loop
endforeach; wp_reset_postdata();