I just got my score on the subject test this week. Scored an 820 (99th percentile). I did major in psych, but I've been out of college a couple years now. I studied both the Princeton review and Kaplan’s prep books. I'd recommend studying Kaplan’s first (it's more intuitive in its format, easier to follow, full of helpful mnemonic tricks). Then I'd fill in the gaps with the Princeton Review's guide- this one seemed to cover a wider scope of information, but in less detail than Kaplan’s, which meant I had to research some of the terms/people from the book online. I also bought a used intro to psych text online and reviewed anything that I felt I didn't know/remember.
I walked out of the test feeling less than confident in my performance, but I think that's just the nature of the test since its scores are normative.