Purchase an Accuplacer exam study guide for math and use it. Almost all of the Accuplacer study guides present the material in a series of modules, beginning with the basics such as simple arithmetic and working up to more advanced concepts.
Work through each module. Spend less time on easier modules and take more time to study the more difficult concepts. Complete each practice question to the best of your ability.
Complete the practice test at the end of each unit. Follow the instructions for taking the test exactly; for example, there are usually time limits and instructions about if and when you may use a calculator. Follow the instructions because if you don't, you won't get an accurate picture of what you need to study or how you will do on the test in a real-world context.
Examine the problems that you got incorrect, and go over those units again. If you still do not feel confident that you know the concepts, then get a tutor. Call the math department of your university and ask for a list of tutors.
Take two or three practice exams before you take the Accuplacer. Mimic the exam environment as closely as possible by limiting your time and distractions; this will prepare you for the exam situation and help alleviate test anxiety on the day you take the real Accuplacer.
Get a good night's sleep before the exam, and eat a good breakfast that morning. You want to be as sharp and awake as possible.
Pace yourself during the exam. Take your time with the first series of questions because the Accuplacer is "computer adaptive," meaning that is you get the first few incorrect you will get easier questions after that and therefore, a lower score. This partially determines your score; if you never get the college-level math questions because you scored too low on the first set of questions, then you won't score in the upper ranges of the Accuplacer.