Want to make changes to success.phtml (Magento’s order success page) without having to place a new order every time? The following temporary modification makes that possible.

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

Important: This temporarily modifies a Magento core file. It is absolutely not a recommended approach for permanent changes. Learn more in this article.