Read the USGBC LEED Reference Guide. This book is slightly over 400 pages, but you'll really need to read and know most of the material in the book to pass the test. Pay special attention to any codes and standards that are referenced, and make sure you know how to apply any formulas used in the book.
Learn the credits. The LEED system is based on a series of credits that are each worth from one to ten points. You must know the intent of how credit and how to apply it in a practical sense. There are portions of the test that involve calculating whether a certain scenario would earn a specific credit, so memorize the formulas you see in the book.
Study the credit templates on the USGBC website. A large portion of the test that is based on implementing the LEED process, which includes filling out credit templates, knowing who is responsible for each, and knowing which documentation is to be included. All of this information is on the USGBC website, but you have to take the time to search through it.
Know the administrative process of LEED. Further study of the USGBC website will teach you about Credit Interpretation Rulings and the USGBCs internal processes related to how a building is certified. You must know all of these subjects for the test.
Take practice tests. Practice tests are a great way to gauge whether you are ready for the real thing. One of the practice tests that most closely resembles the actual exam is published by the Colorado chapter of the USGBC. There are also some free tests on the USGBC website.
Sign up for the test via the USGBC website. The test consists of 80 multiple choice questions which are randomly generated. If you don't pass, try and take the test again soon after so you don't forget everything you've learned.