DON'T bring a backpack/large purse since it won't fit in the small locker space you're given.
If you bring flashcards/prep material, you can look at it before you are checked in to the testing room, but you're not allowing to consult them during the test breaks, so make sure you stow them way in the back of your locker and store your snacks/drinks in front so they'll be no suspicion that you consulted them. To be on the safer side, don't bring any test prep material to the testing center at all, or if you do, leave it in your car.