ACT Study Tip: Take as many practice exams as possible, time them, and then take more!
The ACT is not the SAT, and the sooner you recognize that, the better! When I first set out to study for the ACT, I figured I could do it the same way I had gone about studying for the SAT, but that it wouldn't take as much time—after all, I'd basically already prepared myself for this exam too, right? Nope!
A major difference I wish I had known going into my test prep is that the timing is very different on the ACT, and in particular, the amount of content covered in each section is different (in my opinion, the ACT sections go by much faster, and the material covered is often more dense). Because of this, taking as many practice exams as possible and timing each section is really important. It's the one way to know how much content you can expect to get through in each section. If you realize that you aren't covering all the material in a certain timed portion, pay special attention to the questions that take you the longest so that you can hash out a plan for how to work through them faster or strategize to tackle them at the beginning or end of the timed section. Just as importantly, if you realize that even with a significant improvement in speed, you may still not complete every question in the section, start to think about what your approach will be when it comes to guessing, and how to make an educated guess for each type of question.
On that note, another key difference between the SAT and ACT: the ACT does not penalize you for incorrect answers! It is therefore in your best interest to answer every single question on the test, so don't only consider thinking about how to make an educated guess, but practice guessing. One tip I was given was to take entire practice exam sections only using educated guesses. I did that multiple times, cutting down the time I would allow myself to spend on each question and forcing myself to answer in that time limit as a way of mimicking a situation where time was running short and I still had those questions to answer. This turned out to be helpful on exam day when I unexpectedly ran very short of time on a math section, and had to guess on multiple questions near the end.
A few other differences that are helpful to understand relate to the individual sections themselves. What makes up the SAT verbal section becomes three sections on the ACT: reading comprehension, English, and writing. The writing in particular is not assessed in the same way as the SAT, and is more focused on arguing a point with the information given. Use of representative examples and clear, rational thought development are key. The math sections also cover slightly different material than SAT math: the toughest sections are often trigonometry problems, but there will be fewer of them, and I found there to be a greater emphasis on geometry when I took the test. And, of course, the science section is entirely a new challenge. As someone not inclined toward the sciences, I was nervous about that section, but it's helpful to recognize that the scientific concepts themselves are tested less than your ability to understand the parts of an experiment and apply reading comprehension skills to analyze the experimental processes described. For this section, as for the ACT as a whole, the mantra really is practice, practice, practice. Finding as many actual ACT review questions as you can, timing yourself as you go through them, and learning how to guess, even if you don't think you'll need to do so, will be a great help in mastering the material.