Copy the following snippet and edit the message on line 5. Once done, paste it on your functions.php file and save.
function wptutsplus_text_after_title( $post_type ) { ?>
<div class="after-title-help postbox">
<h3>Using this screen</h3>
<div class="inside">
<p>Use this screen to add new articles or edit existing ones. Make sure you click 'Publish' to publish a new article once you've added it, or 'Update' to save any changes.</p>
</div><!-- .inside -->
</div><!-- .postbox -->
<?php }
add_action( 'edit_form_after_title', 'wptutsplus_text_after_title' );
Thanks to WP Tuts for this great tip!