Here are our favorite, trusted sources to help you make the most of your time pre-PSAT and between tests to address general and specific test challenges. You'll note that a number of big names in test prep aren't listed here. That's because their tests are not similar enough the the SAT to be considered good practice.
College Board practice tests 5-8 only (the test has changed enough over the past couple of years, and these are more like the test you will take than 1-4)
Khan Academy (link your College Board account here and you will get suggestions about what you need to work on once you have PSAT/SAT scores available; note that the math help is much more valuable than reading and writing)
Daily Practice for the SAT (free app from the College Board with daily questions with answers that can be tallied; if you take a full practice test and then upload a picture of your bubble sheet, this app will score it for you)
reddit.com (search for "real SAT tests"; many October, March, and May tests are posted. If you find them, download them because once the College Board finds them, they're taken down pretty quickly)
https://www.erikthered.com/tutor/ (good individual math practice problems and explanations)
The Ultimate Guide to SAT Grammar, (Third edition), Erica Meltzer (great explanations and practice for the writing section)
The Critical Reader, (Second edition), Erica Meltzer (great explanations and practice for the reading section)