If you have to change the design, add a block, or something like that, in your success page. You don’t need to make a whole purchase each time.

There is a workaround for that (it should be just temporal, until you reach whatever you want).

Go to the following file: app/code/core/Mage/Checkout/controllers/OnepageController.php

Check the code near line 240, on the successAction().

There is a function call to session clear, just like that:

You need just to comment or remove this line. Then, you will be available to test your success page all times you want.