The physics GRE is the test you must take to apply to physics grad schools. Its like the SAT but for physics. And just like the SAT, it really is a poor indicator of how you will do in grad school. But, its a hoop you must jump through, so here are some tips and resources.
100 multiple choice questions (5 choices) in 170 minutes. That comes to 100 seconds per question, so speed is the name of the game. The test covers the topics that all undergrad physics students are suppose to have covered. Here is a good overview of the topics courtesy of Wikipedia.
Speed is essential. Try to eliminate solutions before doing calculations. Usually you'll only have 2 or 3 sensible answers left, and only a partial calculation will be necessary to determine which is correct.
Alternate between taking practice exams and reviewing material you did poorly on. If you spend too much time on test questions, you are just memorizing those answers. But if you spend too much time reviewing material, you are probably going into more depth then is needed for the exam.
Make a set of flashcards.
Keep it in perspective. You have to take this test to apply for grad school and it will make an impact on the school you attend, but it really is a poor indicator of your future career in research.
Recommended Study Plan
If you want an extremely detailed study plan, definitely check out the great website from a study group at the Astronomy department of the University of Washington. If I had to take the exam again, I would follow this site closely!
If you want a less detailed study plan, check out my advice below. This is an ideal plan, one that I did not follow but wish I had. This plan is for someone taking the exam in October, so adjust the dates accordingly for a different exam date.
May (5 months to exam): Take a practice test. Force yourself to sit down and take it under test like conditions. I recommend using the 2001 exam. Grade the practice exam.
If you felt good about the exam and liked your score, you can ignore the rest of my advice. You are a genius.
Otherwise, don't reach for alcohol. The point of taking a practice test without any preparation was to provide motivation for you to study throughout the summer.
Review the correct solutions to problems at GRE Physics. Make a note of the topics you missed.
Open up your textbooks from undergrad (you did buy them all right? And didn't sell them back?) and review the topics that you are rusty on (as determined by the past exam results)
June (4 months to exam): Take another practice test under test conditions. Grade it. I recommend the 1996 exam. Repeat steps 3 and 4 from above.
July (3 months to exam): Time for another practice exam. I recommend the 1986 exam. Grade and review.
August (2 months to exam): Time for another practice exam. I recommend the 1992 exam. Grade and review.
Early September (5 weeks to exam): Take your last remaining practice exam. If you followed the above steps, it should be the 2008 exam. I told you to save this for last since it is the most similar to the current exam. Do a very detailed grading, keep careful note of topics you still need to review.
September/October (last month to exam): At this point you've taken all the practice exams, so you hopefully only have a few topics that you still have issues with. Go to the Ohio State website where they have compiled the questions from past exams by topic.
October (less than 2 weeks to exam): Memorize equations. You don't want to waste valuable test time deriving results.
All the exams above have an answer key. However, if you want details on how to solve the problems, you will need to look elsewhere.
My personal recommendation for solutions to the 1986, 1992, 1996, and 2001 exams is GREPhysics.net.