For most people, the biggest barrier to a higher ACT score is the time constraint. Many of my students can answer most of the questions, but they only get about half-way through the practice test before time runs out. For the next practice test, they rush and miss a lot of questions. This is especially true for the Reading and Science sections. This problem calls for a two-pronged attack.
The first task, obviously, is to practice your pacing. Practice reading faster. Skim diagrams. Stop agonizing over each problem; do your best as quickly as possible and move on.
Sometimes, however, you need to concede that you are not going to finish. The best thing to do is to have a strategy for which questions you will answer and which you will skip. Some students are surprised to learn that they can skip their least favorite science passage, take a little extra time with the remaining questions, and be quite happy with their scores. Skipping an entire Reading passage is more costly since there are only four, but for some students that is still the right choice.
Of course, deciding on your personal strategy is very time-consuming. You have to work through several practice tests to try out different timings. Which passage should you skip? If you have time at the end, should you try to make headway in your skipped passage, or should you double-check your answers in the passages you completed? All of these are individual choices. You will need to begin working at least a month before the test. If the test is only a week or two away, you may need to speed up the process by finding a professional tutor.