Study as you go along during the first two years. You will be tested on the entire course of these two years, so it is crucial that you attend all your classes, stay mentally alert for lectures, take notes and jot down the points your professors stress on most subjects and revise for several hours daily. Keep in mind that you are not just studying to pass class tests, but also to prepare for boards and gain sound knowledge for when you're one day a doctor.
Register for the test six months in advance. Make sure you know where the venue is, what to take with you and the timings of the test. Most people take the test around June or July.
Make a study schedule and stick to it. Keep in mind that postponing the test is not a viable option. If you keep the option of postponing the test in the back of your mind, you will not be motivated to study as hard as you possibly can.
Leave out the big books. Even though you will most probably need to study at least 6 to 8 weeks continuously in spite of the fact that you have been studying regularly for the last two years, you don't have time to go through giant books such as Guyton's Physiology, Robbins Pathology or Katzung Pharmacology. Stick to review books. Most publishers make concise editions of these. Also go through Kaplan's First Aid series. Another review book med students swear by is Microbiology Made Ridiculously Simple. You'll need to browse around until you find which books you most comfortable with.
Do question banks. USMLE World is a med student favorite. There are at least 2000 questions. Students who beat the curve do this question bank all the way through.
Take practice tests online. Some sites require you to pay for them, whereas others offer tests for free. Take these tests while pretending you are taking the USMLE. The test will be 8 hours long with 7 sets of 50 questions. You need to practice in order to build your stamina.
Exercise and eat healthily. Prepping for the USMLE requires intense stamina, will power and mental energy. Lack of exercise and eating junk food will wear your body out, causing you to become lethargic, irritable and stressed out. Maintain your health because you're running a mental marathon. Your brain works best when your body is well nourished.
Make sure to be well rested the night before the test. Take snacks with you. You'll be taking an 8 hour long test, so it's important you stay mentally and physically strong until the very end so you don't end up losing points in the final section.