Locate as many study guides and books as you possibly can. Each specific subject comes with its own set of unique questions, so be certain that the materials you find are directed toward the specific test you're taking. Be certain it comes with many practice tests.
If you have old college textbooks and classroom notes pertaining to the subject, dig them out and review them. However, focus more on the study guides, as they'll give you a clearer idea of what exactly you need to know.
Spend a portion of time every single day reviewing all your material. Begin studying at least a month, if not three months, in advance. This will allow you to soak in all the information, as well as determine what areas you are weakest in.
Take many practice tests. In addition, don't be afraid to retake a practice test that you've already taken. Add a level of stress and anxiety to the process by limiting yourself to one hour for each test. You want to simulate the conditions of an actual SAT Subject Test as much as possible.
Familiarize yourself with the rules and instructions regarding the SAT Subject Test. Its format has not changed in years, so by reading the guidelines in advance, you will save yourself some key time when actually taking it.