Magento Failure Page Themeing

Locate This File: /app/code/core/Mage/Checkout/controllers/OnepageController.php

Go to Mage_Checkout_OnepageController::successAction()

Comment the line $session->clear();. 

Now you can make one order and refresh the page as often as you like for styling the success page,
simply put through a test order and once you are on the success page just change the url 
from /checkout/onepage/success/ to /checkout/onepage/failure/

Don't forget to undo your modification to the core code to avoid the death penalty!
More info here if need be: http://magento.stackexchange.com/questions/835/how-to-test-or-style-the-order-success-page/836#836