Would you like to test or style your stores success.phtml without reordering all the time? It can be done using the following temporary change:

Open /app/code/core/Mage/Checkout/controllers/OnepageController.php and look for the successAction() function. Now remove the line $session->clear();.

Important: Please note that modifying Magento core files is considered very bad practice! If you have to modify core files, always override them in your local code pool!