This may be the most obvious of study techniques, but it really is the most important. The Praxis II Social Studies exam covers a wide range of information, so never leave reading/re-reading your text books to the last minute. Besides the books you've used in class, it can also be very helpful to read other history or economics books from bookstores, public libraries or your school library, particularly those that cover topics you have had difficulty with. Try looking for books that have a more literary and less-structured feel to them, as these are often easier to retain and less dull than regular academic books.
Practice makes perfect, and taking mock-Praxis exams or online tests can certainly help prepare you. While plenty of practice tests can be found in books like "Cliff's Test Prep Praxis II," you can often find free ones on websites like PraxisSocialStudies.com or Praxisiitest.com. After taking each test, you can view the correct answers so you can see what you need to work on the most.
Be sure to take practice tests in the same manner that you will take the actual exam. This means timing yourself and pretending it is the real thing. For example, if you know you tend to skip around questions, do so while practicing, but always be sure to circle the question so you will remember to go back to it.
Reviewing notes taken in class is certainly important, but it can sometimes be more useful to take new notes while studying. As you read texts or notes, jot down any dates, names or other information that come up as unfamiliar or important to you. Writing it down will help it stay in your head, and you can use these notes for further review later on, including the morning of test day.
Never skim or skip reading instructions while taking the exam. The Praxis often has different instructions for each section, so not reading closely can result in the loss of otherwise easy points. A good measure to take is to start doing this while you take practice exams, even though you may already know the instructions.
Studying in your room or in a library is not the same as taking a test in classroom full of other people. Imagine what test day will be like, complete with possible distractions. Other test-takers will make noise, and there could even be unavoidable disturbances like wind blowing from a window. Though not necessary for every study session, it helps to take practice tests in a similar test environment. Limiting yourself to the same amount of time you will have on test day will also help you mentally prepare.