Set aside two to four weeks to study for the ACT. You need time to prepare for this test; it is not one that you can cram for. Make sure you have ample preparation time before your test is scheduled.
Acquire review books for each subject. Most local bookstores will have study guides available for standardized tests; look for books specifically made for the ACT. Your local library may have copies you can borrow, if you would rather not purchase the books.
Register for the ACT online preparation. The online preparation can be found at ACT.com and is probably the most important tool you will use while preparing for the test. The online preparation will also allow you to get comfortable with the types of questions the ACT will ask (They can be confusing if you aren't familiar with them). Take the practice test to see a simulation of what your score would be. Use this as a starting point, and try again after you have spent more time studying.
Start an ACT study group with your peers. The goal of this is to have several students in a group preparing together for the ACT to exchange advice and offer suggestions for improvement. Practice the Mathematics and English basics with flashcards to help you remember the rules. Use your review books to improve your critical thinking and logical skills to help you with the Science and Reading tests.
The night before the test is one of the most important times of preparation. You don't need to cram; you have been preparing and studying for several weeks. It is important to relax and let your mind and body rest the night before the test. Spend about an hour reviewing and take the rest of the night off; but remember, no partying and no excessive junk food. You need to be well-rested and alert for the ACT.