Take the practice tests
Take the official SAT or ACT practice tests. Take them with the given time frames—no bonus time. Take them in order with only the break time that you’d get on the real test. Make the practice test as close to the real test as possible. Nothing will prepare you for the real life test than the real practice tests.
Know the location
Know where the test is being given. Don’t just know that it’s at “City High School” but where at City High School? Where do you park? Which entrance? Even if you think you know the location, you can’t be too sure. If it’s a place that you’ve never been to, take a road trip and check it out. This sounds simple, but nothing raises blood pressure on test day when you can’t find the right place.