Whether you're taking the SAT, the GRE, or the TOEFL, the harsh fact of the matter is that you'll be undergoing an endurance test. Such standardized exams aren't merely a measure of your verbal and mathematical ability; they're also a test of your stamina and willpower.
All of these tests are somewhere in the neighborhood of four hours long. If you're tired during the first hour, it's hard to imagine how well you'll do during the final three. If you're pumped up with nervous energy at the beginning, but feel yourself crashing by the halfway point, you're toast.
Takers of the computer-based GRE are allowed a ten-minute break after the first couple hours; it's the only official break (other breaks may be possible by special request, but I don't think they stop the clock for you). High schoolers taking the SAT I will normally get several short breaks (about five minutes) and one long break (about 30 minutes). For TOEFL takers, ETS gives one sanctioned break and allows testers to take additional breaks without stopping the clock.
Obviously, most testers will prefer to tough it out and stay at their desk except during the mandatory breaks. This means that, before the test, testers should get themselves mentally and physically ready for the ordeal ahead.
The most common and long-standing piece of advice is to sleep well the night before the test. Another bit of practical wisdom is to work on practice problems just before going to sleep so as to be in the proper frame of mind on test day. Manhattan Prep's GRE blog also recommends watching one's nutrition: a tester should eat well and go for complex carbs and natural sugars on the morning of the test: oatmeal, wheat bread, fruit, etc. will break down slowly, releasing energy in a steady stream, thus reducing the chance of a high-and-crash, such as when one eats processed sweets (think: candy, Pop Tarts, etc.).
What you do before the day of the test will affect your performance on the test. If you cram the day before, sleep fitfully the night before, wake up tired and anxious the morning of the test, and feel yourself drowsing (or becoming generally unfocused) during the test, you aren't doing yourself any favors, and you shouldn't be surprised at your low score when test results are sent out.
Keep in mind that knowledge isn't enough for these standardized tests: endurance matters.