Page.php

<?php

class Page extends SiteTree {
	
}

class Page_Controller extends ContentController {

	/**
	 * @return ArrayData|false
	 */
	public function FlashMessage() {
		if ($flashData = Session::get('FlashMessage')) {
			Session::clear('FlashMessage');
			return ArrayData::create($flashData);
		}

		return false;
	}

}

Page.ss

<% if $FlashMessage %>
	<% with $FlashMessage %>
		<p class="message {$Type}">{$Message}</p>
	<% end_with %>
<% end_if %>