Purchase an ASVAB study guide from any major bookseller. Start with the preliminary assessment exam contained in most books. This will give you a sense of your strengths and weaknesses among the test's subject areas, which include mathematics, vocabulary, reading comprehension, science, mechanics, electronics and auto shop.
Develop a plan of study focused on improving your weaknesses. A self-study guide is a good tool for reviewing test strategies and practicing exam questions.
Enroll in an ASVAB review course. These are available both online and in classrooms from numerous credible, national companies. For people who have difficulty directing their own course of study or who prefer to ask questions, an instructor-led course can be more effective.
Practice for several months before taking the exam. Get an exam schedule from your local military recruiting office, then plan for a date at least three months out. You want sufficient time to study to be as competent as possible in each subject area, but don't take so long that information and strategies are no longer fresh in your mind.
Guess when you don't know the answer during the ASVAB exam. Wrong answers do not count against you, which means a guess can only work in your favor. Make it an educated guess whenever possible. Additionally, eliminate any answers you can identify as wrong before guessing among possible answers.
Retake the exam if you aren't satisfied with your score. The military requires one month between exams and allows at least three exams per year. Each score report is a tool to help you focus your study and continue improving. High school students can take the exam as early as their sophomore year, but scores are only valid for military applications from age 16 onward. Military hopefuls can take the exam several times during their junior and senior years of high school to try to boost their scores. The military will honor their best scores during the application process.