Test yourself with a prep-test. It shows what your strengths and weaknesses are, and what items you need to learn. There are several test materials available for free on the ACT's website or you can visit your local library.
Take note of the areas in which you are weakest and things you need to learn. You could even put them on flash cards.
Study the weak areas for several weeks. Even give them a quick review the night before the test.
The day before check and collect your supplies such as your admission ticket, identification, pencils and calculator.
Get a good night sleep. It's important to be well rested so you can focus on the test.
Eat a good breakfast and dress comfortably for the same reason.
Carry snacks. You will need them to keep refreshed and alert during the breaks.
Don't linger on one question or one section too long. Part of the ACT is time management so try going through the test as quickly as possible. Answer the easiest questions first, skip the questions that are too hard, then when you have time, go back and answer them. The goal is to keep going.
Answer all the questions, even if it means guessing. There is no penalty for guessing, so take a chance.
When in doubt, eliminate answers that you know are incorrect, thereby increasing your chances of getting the correct answer.