Take a variety of math courses in high school. Consistent study over several years can be invaluable, in terms of scoring well.
Review sample questions from the official ASVAB website.
Practice basic math skills, such as working with percentages, roots, exponents, algebraic expressions and geometric formulas.
Complete the practice questions on the online ASVAB study guide.
Move through the questions without getting bogged down. If a question seems difficult, skip it and return to it later. Narrow your choices if possible, and make your best guess, if necessary. You aren't penalized for guessing.
Rest adequately the night before the test, and eat well the morning of the test. Rest and nutrition are important in order to concentrate.