Make sure you know the subject matter
GRE Subject Test questions are designed to measure skills and knowledge gained over a long period of time. Although you might increase your scores to some extent through preparation a few weeks or months before you take the test, last-minute cramming is unlikely to be of further help. In addition to downloading the practice book, a general review of your college courses is probably the best preparation for the test. However, the test covers a broad range of subject matter and no one is expected to be familiar with every question.