Since the tests concentrate on subjects found in all schools, most students are well prepared for the examination by the time they reach 11th or 12th grade. If you haven't taken a math course recently, or if you don't remember your basic English usage, reviewing the appropriate areas will help. However, studying specifically for the reading test or the science reasoning test may not be very fruitful. In fact, it's hard to know exactly what material would be worth studying.
Overall familiarity with the tests not only reduces test anxiety, but enhances performance. Self-assurance, the feeling that you will do well, helps too, but be wary of overconfidence. Regardless of how ready you think you are, remember that it's impossible to be too well prepared for the SAT or ACT.