Summarize:

There are numerous ACT preparation guides and practice test workbooks and websites available, most of which must be purchased of course; you can also find free resources as well, though. The official ACT website offers official prep tests and a printed prep guide for purchase.  If you're looking for free help, if also offers practice questions and a free “Question of the Day”. Set aside a specific time each day for ACT prep, starting well ahead of time (perhaps up to six months ahead of test day).  Try to replicate the test conditions while you prepare.  Find a quiet place (try playing classical music if total silence bothers you) and use a desk/table and chair.  You won't be sitting on a couch when you take the test. Focus on one subject at a time.  Perhaps do one topic per day.  You will be answering questions one subject at a time, after all. You want to be ready for all parts of the exam, but it you struggle with math more than the other sections, for example, dedicate more time to it.  Focus a little extra study time and a few more practice tests on it.  Don't give up on a section in which you struggle.  Even being able to give more educated guesses will help -- remember, you are not penalized for guessing on the ACT. Some experts contend that the ACT has a more challenging math component, while the SAT has a more challenging vocabulary component.  So, if you are comparatively strong or weak in one of those areas, and have the option of taking either test, you may want to consider switching from one to the other -- or taking both. For most people, the best way to prepare is to take practice tests -- lots of them.  Take practice tests specific to the ACT so you can study and get comfortable with the format at the same time.  Try timing yourself when you take a practice test section (perhaps do one section a day).  If you don't finish within the test time limit, keep going until you do finish to give yourself a better idea of how much time you need to shave off your test-taking for the real deal. As the test day approaches, try taking your practice tests at the same time of day as you will take the real test.  Prepare your mind and body for test time. Practice these maneuvers until they are like a reflex, so you won't have to stop and think about doing them on test day.  Keep close track of time.  Be rigid.  Use a timer.  There is no wiggle room on test day. Skip over difficult questions.  If you don't understand or know the answer to a question almost immediately, leave it for the end of the test time.  But make sure you leave time to at least guess on blank questions at the end.  Remember, there is no penalty for wrong answers, and you'll probably luck out and get a few right by guessing. Practice your neatness.  This may sound silly, but you need to be able to fill in the multiple-choice bubbles neatly and completely to ensure that your answers are properly recorded by the scoring machine.  Use a pencil for your practice exams and train yourself to fill in bubble sheets quickly but neatly.  Find some extra bubble sheets and practice just your technique on those if necessary. If you finish practice sections early, make use of the extra time to go over answers and tidy up your bubble-sheet filling.  You won't be able to move ahead to the next section on test day, or do anything else but wait for the time to expire, so get in the habit of maximizing every second you have on each section.
Acquire study guides and practice tests. Find your studying groove. Target your weak spots. Take practice tests regularly. Employ testing-day strategies while taking practice tests.