Properly preparing for the ACT is the most important step to achieving a high score. Make sure you allow yourself several weeks for review prior to the scheduled test date. Don't try to cram the information, because the ACT isn't designed for that. The ACT is a general overview of your major subjects; understanding the subjects is more important that memorizing the facts. Concentrate your efforts on studying in the weeks leading up to the test. You will need to review basics for Mathematics and English, and make sure you have a broad understand of critical reading and logical skills (for the Reading and Scientific Reasoning sections).
Make sure you get a good night's rest the evening before the test. Go to bed early and wake early to make sure you are fully functional at the exam. Eating a wholesome breakfast that is high in protein and low in fat will help you concentrate during the exam.
Being comfortably dressed is an advantage when taking the ACT. You will be sitting at a desk for approximately 5 hours, with very limited breaks. Avoid tight clothing; try to dress in loose clothing, if possible. The more relaxed you are during the testing, the more you can concentrate on the test.
When taking the exam, don't spend too much time on a single question and don't rush through the test. You are given ample time to complete each section of the exam. The best strategy is to answer the questions you know first; and then return to try to complete the harder ones. It is highly unlikely you will get every question right, and you are not expected to.
You will be notified when you have 5 minutes remaining on each section; at this time if you have several unanswered questions it is recommended to make educated guesses, and with the multiple choice tests, you have a strong possibility of getting some right.