I am wondering if there are any special segulos or tefillos that one can say if he wants to have success - this example, a big test. The segulah/tefillah could be related to knowledge, but a segulah for success in general works too, i.e., if someone were to daven for success in an upcoming interview.
Remember that you have to do your worldly effort, and that God is in control. Sorry, no magical potions or trinkets here.
I would think Psalm 90 is a fitting one to say; it reminds us of how small we are, asks for "a heart of wisdom", and concludes with a request for our efforts to prosper.