Sign up for a testing date as early in advance as possible. This will give you the maximum amount of time to prepare, and give you plenty of time to fix things if there's an error in processing your registration.
Study the test material thoroughly by taking advantage of whatever resources are available to you. Take a strategy course or purchase a practice exam manual. These types of services exist for many national exams, including the SAT and ACT, LSAT, GRE and MCAT. Kaplan and The Princeton Review are two of many companies that offer test prep services.
Get to know the testing procedures. Take practice exams to familiarize yourself with the format, and understand how the test is scored. This will help you choose an effective test strategy. For example, for many exams, there is no penalty for guessing. For other exams, like the SAT, you receive a small penalty for wrong answers, so you need to answer more carefully. If your exam includes an essay, practice writing clearly while timed, and find out the type of information you need to include.
Take care of your body the week and night before the test. You won't do as well on a test if you're tired, hungry or hung over. Get enough sleep, drink plenty of water and eat a reasonably proportioned and nourishing breakfast the day of the test. Use caffeine sparingly. It'll certainly perk you up, but it also can make you nervous and jittery if you have more than your body is used to.
If you're trying to get into a good school, you may want to take a standardized test multiple times to ensure your best score possible. Most schools value your test results as part of your overall application, and will often only look at your top scores.