The first thing to do is to paste the following snippet where you’d like the “Pin It” button to be displayed. Note that this code must be inserted within the loop.
<a href="http://pinterest.com/pin/create/button/?url=<?php the_permalink(); ?>&media=<?php $thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'thumbnail' ); echo $thumb['0']; ?>&description=<?php the_title(); ?>" class="pin-it-button" count-layout="horizontal">Pin It</a>
Once done, open your footer.php file and add the pinterest JavaScript code:
<script type="text/javascript" src="http://assets.pinterest.com/js/pinit.js"></script>
Thanks to Douglas Karr for the cool code!