I've seen some sights that have ads or content between Wordpress posts, for example on this blog: http://www.womma.org/blog/ there is a form to sign up for their email lists between the 2nd and 3rd post. How do you do that?
You need to edit your page.php file to include the content between posts. Just watch out for The Loop when you do so. (And please consult the WordPress Codex.)