I find that the best way to prepare for these exams is to simply take the most challenging courses available at your school. I went the tutoring way, took those prep classes from Kaplan, and studied SAT books, but none of that stuff worked.
By taking the hardest English classes however, I was able to more critically comprehend literature because I was expected to read the material daily, soak in the material, and then discuss it in class. Prep books and classes will never allow you that much control over literature. It also improved my writing substantially because reading and writing go hand in hand. Many times, I would be expected to critically read a passage and then write an essay on that topic. So for the exam, I would suggest for the writing section of the SAT, to go into there with a wealth of references. Use the books you read from class, current news, movies, and the daily media rather than simply just your personal experiences.
For the math section of the SAT, again, taking the highest math classes allowed me to look at different ways of solving problems. Math has one correct answer...however, there are many times when a problem offers multiple ways to reach that answer. I didn't realize this until I took higher math. I mean sure, the SAT only tests up to Algebra II, but that doesn't mean you should stop your math career there. By continuing to take challenging math courses, you are developing your brain more and more, pushing it to find newer ways of solving problems. Remember, the SAT tests how you apply your knowledge...not how much you know.
Now as for the ACT, I have no idea how to approach. I only took one practice test and did far from awesome on it.
Good luck!