I also had a background other than psychology. I have a really hard time with memorizing and I got above 80%. I would suggest not to waste your time trying to knock it out of the park-- if your other credentials and general GRE are good, an adequate psych GRE score should be fine. In my impression, it is used mostly to exclude students who do really poorly and clearly don't have enough background in psychology, and an amazing score isn't the deciding factor that's going to gain you admission. (This is just my impression, maybe someone else knows otherwise?) So, as long as you do adequately on the test in April, I recommend not retaking it in October (unlike the general GRE, which I think is worth retaking once if you think you can go from adequate to excellent).
I used the Kaplan review book to guide/ organize my study and supplemented with textbooks/ internet to understand the material. I also skimmed the personality section of the DSM-5.
Great decision to start studying early! I ended up cramming while I was writing applications in the fall and wishing I had taken the April test.
Here's what I wished I could have done with more time:
1. Dedicate a week of studying to each section on the test (not the whole week, I mean a few hours during that week). Read the introductory chapter of your textbook on that section, access online materials (I found watching videos helpful for giving overviews of unfamiliar topics-- but use whatever medium helps you learn best), make sure you understand the material, and make a list of terms/ names to memorize.
2. In the few weeks before the test, review each section with a focus on memorizing (using flash cards or whatever works best for you).
3. For the most challenging sections (for me, this was memorizing the physiology terms like parts of the brain, functions of the neurotransmitters, etc.) study as you would for the other sections, but then cram in the few days before the test. I know that cramming is frowned upon, but there were a few details I memorized the morning of the test that actually appeared on the test. I wouldn't have remembered them had I memorized them a week earlier, and I don't remember them now
I disagree somewhat that taking a lot of practice tests is helpful, unless you have a particularly hard time with test taking in general. Practice tests are helpful for the general GRE, which tests skills and for which you can learn strategies from practicing. Other than the experimental questions (which you should practice), the Psych GRE mostly tests knowledge, so I think your time is best spent acquiring that knowledge. However, I definitely recommend taking the practice test provided online by ETS twice (once after you've done some review and then again in the week before the test) and knowing all of the material on that test.
Good luck!