For me…
English
Read Barron’s ACT 36. Write down all of the grammar rules written in the book in your own words, then study it. That alone improved my English score from a 29 to a 32.
Math
Assuming you have already taken practice tests, rework the math problems you missed. Then, write down what you would have needed to know (formula, concept, etc.) and go over that before the test
Reading
My score doesn’t give me the right to give other people tips.
Science
If you are confused with questions about graphs/tables, read the axis titles and key. Also, physically put your finger on what the question is asking you to look at. This helps zero in on whatever you are looking at.