Hmmm, its like the SAT's, not a whole lot you can do. My $0.02, take a look at the questions in the aero, nautical, naval section that you didnt know from the practice test, and try to understand them. There was a question that mentioned the Bonhomme Richard, never even heard of her before so I looked it up. Ended up being a question that mentioned the very subject on the ASTB...was glad I took the time.
All and all, have a photo ID, a timer of some sort, and a way of verifying your social (SS card, Draft card, etc...), go in with a clear head and an empty bladder and come out knowing you put the test in its place.
Best of luck.