Review the topics that give you the hardest time. If you have a hard time remembering the process of photosynthesis, or if you have always struggled with algebra, spend extra time studying these subjects. Start studying at least three months before your scheduled test, so you have enough time to review all of the information.
Take practice tests. You can find practice tests online, sometimes for a price, that will help you gauge what you need to review. Use ACT-specific flash cards to quickly review information. Set aside any card you don't immediately know the answer to. This will help you zero in on what information you need to spend more time studying.
Take care of yourself. The test is about four hours long -- five if you choose to take the optional fifth portion, writing. So, about two weeks before your exam, get yourself on a rigid schedule of sleep and proper meals. The better rested and fed you are will help ensure you are on top of your game the day of the test.