Familiarize yourself with the test. Take a practice one online. Buy a book of practice PSATs and try to take at least one a week to get used to the type of questions as well as the length. Like running a marathon, part of passing the PSAT is simply having the endurance to complete as much as you can of the exam.
Learn strategies for answering the questions. With this exam, it's better to guess than to leave a question blank, even though you'll lose some time filling in the ovals. There are also strategies for eliminating wrong answers or at least narrowing down your choice to two possible correct ones--read a PSAT prep book or take a course to get a handle on all of these.
Find a tutor or a prep course if you consistently fail practice exams. The scores you get when you take a practice test generally will be several points higher than what you'll end up with when you're in actual, nerve-wracking testing conditions, so if you're on the border, get some professional help before taking the PSAT.
Prepare for the PSAT the night before by relaxing. Clear your mind by watching a movie, television or reading a book--anything to get your mind off the test. At this point, taking a practice exam probably does more harm than good because you might tire yourself out and not perform well the next day. Exercise or yoga also is a good idea.
Eat a breakfast filled with healthy carbohydrates and protein, like oatmeal with peanut butter or eggs and toast. Pack a protein bar to snack on when you get a break. Take your test with a confident attitude--and the reassurance that even if you don't do well, you still have plenty of time to prepare for the more important exam, the SAT.