Don't answer questions that you don't know, they penalize you 1/4 of a point (or they did when I took it years ago) for missing problems. It is best to narrow choices if possible, so that your chances of randomly guessing questions you don't know are 1 out of 3. Obviously answering the questions you know is a sure fire thing. When I took the Physics GRE they didn't allow calculators, and I really needed to figure out √17 to find the depth of a well... and sure enough I needed it out to four digits. I had all of the physics worked out - time for stone to drop, the time for the echo to return... but √17 = 4, = 4.1, =4.12 wasn't good enough to narrow the choices.
Learn how to do square roots by hand if calculators are not allowed. There were also lots of historical minutia that I hadn't paid particular attention to while studying the problem solving side of physics - who discovered what particle, who won the Nobel prize for such and such on said date. And let's not forget about the quantum mechanics from hell. None of the prep material seemed to be similar. It does seem to test what you know, and there isn't much prep you can do on short notice to make up for lack of years of paying attention to the details. In hind sight, reading your undergraduate textbook cover to cover might help a bit. Other than that make sure to rest prior to the exam. Sadly I did mine after working all night, taking the general GRE in the morning, having a big lunch, and then in the afternoon attempting to stay awake for the subject exam.