I was on the PRITE editorial board for 2 years, so I can reveal a bit behind the curtain.
First, I would agree with trying to figure out how, if at all, your PRITE scores affect your standing in your program. At my program, they were entirely irrelevant assuming you weren't in the <10th percentile. Beyond that, no one really cared. More than anything, it was a way to get a very rough sense of topics that you may want to study a bit more on.
The test is constructed to cover a variety of topics, however diagnosis and treatment of psychiatric conditions are the largest topics. The cornerstone reference texts (especially Kaplan and Sadock) are the primary sources for question material, so if you really wanted to go nuts, going through a reference text like that with a focus on topics that you don't know about would probably be "the gold standard." There are always going to be completely off-the-wall questions included on the exam that you either know or don't (e.g., very arcane topics, historical questions, etc.) that aren't worth trying to study for.
Ultimately, I think the best preparation is just trying to learn as much as you can about your patients and read things relevant to them (e.g., reading about schizophrenia diagnosis, epidemiology, and treatment strategies when you have a patient with schizophrenia). Actually pay attention on your rotations and be actively engaged in learning as much as you can. Supplement with past exams if you want to actually "study," but I never specifically studied for the PRITE and did pretty well. I thought the board exam was easier than most of the PRITE exams quite honestly.