If you scored well on certain areas of the exam, focus on your problem areas for the next time you sign up for the test. For example, those who score well in English and reading should remember to focus on math and science when they retake the test. It may be more difficult to force yourself to study the subjects that are harder, but if you already know you can do well on certain area, don't bother spending equal time studying each subject. Instead, narrow your study time down to the subjects in which you really need to improve SAT and ACT scores.
That being said, don't speed through your easier subjects on the exam. Each subject is timed separately, so take advantage of the full time allotted to answer each question carefully. You could still improve your scores in your strong areas as well, so make sure to give yourself a quick refresher in those topics before the test date arrives.