Ideally Juniors should take the PSAT in the fall (OCT 15, 2011). Serious college bound students (4 yr schools - community college students do not need to take the SAT or ACT) Juniors should take their first SAT or ACT in the spring of their Junior year. This allows time for students to retake the test in October if they are not pleased with the scores from the first test.
Statistics show that students increase their scores from 50-100 points with the retake.
What to bring with you on test day. Click on College Board image.