Prepare early. The MSA is given at the same time and asks the same type of questions every year, so you won't find yourself surprised by anything on it if you take your time to get to know the test. Get to know the test inside and out; teachers know the tests well, and will provide you with sample questions in class.
Practice and improve upon your reading and writing skills. The MSA examines both your ability to interpret and understand reading selections, while also prompting you to construct your own essays. Over the course of the year, read magazines, fiction and newspapers, and test yourself for reading comprehension.
Ensure that you understand the required math concepts. The MSA, depending on your grade, tests algebra, statistics, probability, geometry, math patterns and number concepts. Learn all of these well before examination day.
Avoid anxiety. By preparing both in school and on your own time, you'll spend enough time with the MSA that test day won't seem like a big deal. Get a good night's sleep, eat a hearty breakfast and avoid late-night cramming.
Once you've completed the MSA, try your best to keep it off your mind; your score is out of your hands at this point, so there's so reason to fret over your performance. You've done your best to prepare, so you should have no regrets.