It doesn't seem fair that after 11 years of hard work in elementary, middle and high school that one test on one day can have a major impact on not only the type of college you can expect but also the amount of financial aid that colleges will offer. It puts a lot of stress on high school juniors and seniors; however, there are steps that they can take to take some of the anxiety out of the situation. The key is to plan and be prepared. The first step is to know what is expected of you on the test. A good way is to take the PSAT and the SAT in your sophomore year. The PSAT can qualify you for scholarships through the National Merit Foundation and they can give you a good indication of how you would fare on the SAT. There are a host of practice tests in local bookstores and online at collegebord.com. 

Look into taking a SAT preparation course. The course will teach them test taking strategies and what type of questions to expect. The SAT is a test set up to measure reasoning ability not what has been memorized over the last 11 years. While there has been a writing section added to the test recently, the good news is that many colleges don't consider this score the bad news is that many competitive colleges use this score. 

Plan to relax the night before the test, this doesn't mean parties or going to work. You need enough sleep to feel rested and have time for a good breakfast. Four hours is a long time and you will be using up a lot of brain power.

Make sure that you have all the necessary supplies. Bring at least two number 2 pencils, a good eraser, a calculator with fresh batteries, a bottle of water and some snacks. As I said 4 hours is a long time.  Put these in the car the night before to avoid forgetting them in the morning rush.

Wear a watch so that you can time yourself in each section. Sometimes the proctor doesn't write the time on the board.  Allow extra time for any unusual circumstances. If the test is at your high school don't depend on your daily commute time. You never know when there will be a traffic accident or a rush hour snarl. In most cases you will be traveling later in the morning. Allow yourself to be at least 15 to 20
minutes early.

Finally, relax the time will fly by and you will be done before you know it. If you do your best that is all that anyone (including Harvard) can ask.