Study the test format for the test you will be taking. Different entrance exams have different formats. For example, the SAT has grid-in, or student-produced, math problems in addition to multiple choice questions. The ACT has exclusively multiple-choice questions. When scoring the tests, the SAT subtracts 1/4 of a point for each wrong multiple choice answer while the ACT simply gives a 0 for wrong answers. Knowing the test format will help you construct the best strategy for each test later on.
Refresh basic math concepts. Both the SAT and the ACT test basic high school math concepts such as algebra, number properties, geometry, and word problems. Study your notes from earlier classes with a guide to the test, or look at material on the test maker's website to solidify your knowledge of these areas. Even if the material seems too easy while you are reviewing it, the hardest problems on these tests are difficult because they combine basic math concepts into one question, not because they test advanced calculus or other similarly challenging concepts.
Work your way through harder problems. Many guides to the SAT and ACT will list problems by difficulty, so you can start with easier problems and work your way to harder problems. As you do, be sure to study which problems you answered incorrectly and review the correct answer choices, going back over any relevant basic materials you need to review.
Develop a strategy for the test based on your content review. Knowing the test formats, construct a plan for test day. If you have a hard time answering geometry problems on the SAT, for example, you might start with the other problems and only answer the geometry questions if you have time in order to avoid losing points for wrong answers. However, on the ACT, you might just fill in random bubbles if you have not answered all of the questions at the end of the test section, as guessing can only help you and will not negatively impact your score.