Display related posts by category on single-post template...

related-posts.php

<?php ?>
<--! Include inside the post loop - After entry content or comments is good ----->
<div class="related-posts">
<?php 
    $orig_post = $post;
    global $post;
    $categories = get_the_category($post->ID);

    if ($categories) {
        $category_ids = array();
        foreach($categories as $individual_category) $category_ids[] = $individual_category->term_id;
        $args=array(
            'category__in' => $category_ids,
            'post__not_in' => array($post->ID),
            'posts_per_page'=> 2, // Number of related posts that will be shown.
            'caller_get_posts'=>1
        );
        $my_query = new wp_query( $args );
        if( $my_query->have_posts() ) {
            echo '<div id="related_posts"><h3>Related Posts</h3><ul>';
            while( $my_query->have_posts() ) {
                $my_query->the_post();?>
                <li>
                    <div class="relatedthumb"><a href="<? the_permalink()?>" rel="bookmark" title="<?php the_title(); ?>"><?php the_post_thumbnail(); ?></a></div>
                    <div class="relatedcontent">
                        <h3><a href="<? the_permalink()?>" rel="bookmark" title="<?php the_title(); ?>"><?php the_title(); ?></a></h3>
                        <?php the_time('M j, Y') ?>
                    </div>
                </li>
            <?php
            }
    echo '</ul></div>';
    }
    }
$post = $orig_post;
wp_reset_query(); ?>
</div>