@richardpierre I’ve never done this as I have my opt-in form on the sidebar, but here are a couple ideas: 

• You could paste the code at the end of each post in the HTML section of the WYSIWYG editor. 
• Look for a shortcode plugin that allows you to post a shortcode after your posts which automatically pulls your Aweber code into it (I think you could use Shortcode Exec PHP for this)

Hope this helps a little, keep us posted on what you find that works.

@richardpierre It all depends on whether you are using a self-hosted version of WordPress (on your own domain) or using WordPress.com.  Also, the theme you use will be a factor.  Could you specify the WordPress version and theme you are using?

@richardpierre I’ve never used Woo Themes, but I assume it is a pretty standard theme.  You will want to do what @mike-stelzner suggested.  Go to your dashboard > Appearance > Editor, and in the right hand side, look for the Single Post template (single.php).  

Somewhere in that template, you should see <?php the_content(); ?>

After that, add two line breaks <br /><br /> and then try adding the Javascript Snippet version of your Aweber form which should look like <script type=”text/javascript” src=”http://forms.aweber.com/form/01/123456789.js”></script> and then add another <br /><br /> after it.  That *should* put the form right after your post content and before the comments box. 

In the off chance that your theme doesn’t like it and you can’t get back into your dashboard (shouldn’t happen, but just in case), make sure you have a fresh copy of that single.php file and access to FTP ready.  You can upload the fresh copy over the copy you just edited under the wp-content folder, themes folder, your theme folder.  Once you overwrite the edited version with the original version that you downloaded, everything will work again.