How do I add a promotional message to my posts?

You’ll want to hook into the_content to add a message.

Something like this:

<?php
add_filter('the_content', 'wpse51338_filter_content');
/*
 * Filter the content to add a message before and after it.
 */
function wpse51338_filter_content($content)
{
    // not a singular post?  just return the content
    if(!is_singular())
        return $content;

    $msg = sprintf(
        '<p class="special-message">%s</p>',
        __('Here is an awesome message!', 'your_textdomain_string') // this is your message
    );
    return $msg . $content . $msg;
}

As a plugin.