Resist the temptation to study. At this point, you know what you know, and trying to cram at the last minute is counterproductive.
Organize all necessary materials, including directions to the testing location, and put them in your backpack. Nothing is more unnerving (or more preventable) than a last-minute morning panic about number-two pencils.
Choose your outfit for testing day, and lay out all the pieces.
Get a good night's sleep. Easier said than done, perhaps, but at least getting into bed and turning out the light, rather than pacing or watching "Friends" reruns into the wee hours, gives you a fighting chance at some rest.