Start preparing early, and finish early. I recommend you start reading the original study material at least 6 months before the exam. You may assemble the reading materials from the original publications instead of the GARP package. The material is exactly the same, with the exception that the GARP exam preparation package already has all the relevant chapters and papers bundled. If you do not mind to jump between chapters in a book and organizing published papers yourself, then you can find all the core readings on sites like Amazon or in your library. Make sure you have at least two months before the exam to review and solve practice questions.
Take notes while reading. Instead of simply reading the textbooks you should take notes from the beginning. This will expedite your review process greatly. Memory consolidation only works if the information is repeatedly recalled (active recall), and reading alone will not make you remember the relevant concepts well enough for a challenging certification such as the ERP.
Regularly review what you have learnt. Review your notes, diagrams, earmarked pages etc. every few days or at least every weekend. This will help you keep perspective and reinforce the most salient concepts.
Explain to others what you have learnt. If you can, study together with other ERP candidates. This will make sense once you have read and summarized the core readings. Explain certain concepts to each other, or solve practice questions together. Who would not want to know how exactly nuclear energy works? Explain it to your parents or a patient neighbor. Discussing the concepts will greatly enhance your understanding and performance on exam day.
Solve as many practice questions as possible. In order to drill the concepts, you must stimulate your brain. If it feels difficult, then it works and you are learning something. Try to find as many practice questions for the ERP exam, or if possible, enroll in a class to take a practice exam, such as the realistic ERP practice exam.
Take a day off work before the exam. You should not go into overdrive right before the exam day. Take a day off, sleep enough, take a walk, or go swimming. Go to bed early and rise early. This will help you achieve peak performance during the exam.
Research your exam center. Go there once before the exam, find out where parking is located, if the exam is taking place on a university campus, find out in which building it will be taken. I took the exam at UCLA. The campus is a maze stretching over several city blocks. There were no signs whatsoever. As it was early on a Saturday, there were almost no students around to ask for directions. Make sure you know where you have to be on exam day, and if you have questions, ask GARP early.
Bring your own lunch. On the exam site, there may be a cafeteria, but you should bring your own lunch. You never know what they are serving, it may be crowded and you have to wait in line. Instead, prepare a healthy sandwich, find a quiet place and relax during the lunch break. Make sure you bring a beverage and snacks during the exam, as there is probably no drinking water provided in the exam room. The official statement is that there are no food or drinks allowed in the exam room, but of you quietly place a bottle of water on the floor next to your chair, nobody will complain.
Do not engage in discussions about the curriculum before the exam. Save this for afterwards. The exam will be hard enough. You need all mental power you can muster, and you should not waste any energy discussing last minute questions right before the exam. This is why you should have a study group where all questions are cleared beforehand.
Take it step by step during the exam. Time is usually not an issue. Solve question for question, and if you see something you did not anticipate, do not despair. Make a mark in your exam book on questions you are not certain about, and re-attempt them later. Do not leave any questions blank, always fill in an answer. You can always correct your answer later.