If you are trying to style the success.phtml page but get frustrated with having to do multiple orders just to check out, try this trick. Also works great if you are attempting to add in conversion tracking pixels (e.g. AdRoll, Proclivity, Commission Junction, PepperJam, Google Sale, etc.).
// File: app/code/core/Mage/Checkout/controllers/OnepageController.php:291 // Comment out the following line and revert when you are done testing. $session->clear();