The biggest mistake juniors (and their parents) make on the PSAT is NOT SIGNING UP FOR IT!!
The PSAT is your one-time-only opportunity—in early October of your junior year—to take the PSAT and qualify for National Merit Scholarships and many other scholarships sponsored by different organizations and targeting different groups. My National Merit Scholarship, which I earned with my high PSAT scores, got me a full-tuition scholarship to a private university.
For the parents out there, wondering about how to pay for college, let me emphasize that:
PSAT --> College scholarship money
Even if your scores are not high enough for the National Merit Scholarship (about the top one half percent), you can still qualify for other scholarships based on your scores or, at the very least, bring yourself to the attention of colleges and universities everywhere who might be interested in you.
As an SAT tutor, I am always dismayed by the number of SAT prep students who come to me having already missed their chance at these scholarships.
The second biggest mistake PSAT (and SAT) students make is not preparing well enough ahead of time for the test. To get the highest scores, you need to not just learn, but also practice, new ways of taking the tests so that you will actually take the real test this way and improve your scores. Taking the test the same way will get you almost exactly the same results. Taking the test a better way? Better results.
It's not hard, but it does take some preparation ahead of time, and also some individual practice, to make the better way a habit.