How to test or style the order success page - http://magento.stackexchange.com/questions/835/how-to-test-or-style-the-order-success-page
You can temporarily hack the core while you are developing:
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.