Look for a study guide that includes at least three full-length practice tests that follow the format of the official test. "The Real ACT Prep Guide" is recommended on the ACT website and provides previous actual tests for practice. The practice tests should include the optional writing test if you need to complete that section.
Choose a study guide that gives a summary of test subjects if you need to review the important information in math, English, science and writing before completing practice problems.
Check that there are practice problems throughout each lesson that are representative of the actual test.
Get a study guide that explains why answers choices are right or wrong for each practice problem.
Review online prep programs at the official ACT website and other websites. This is a good secondary resource for students prefer computer-based learning. Study guides are also available on CD if you have limited Internet access.