1. Get the official SAT Study Guide from the College Board. This book contains 10 actual old SAT tests.
2. Review the book.
3. Take a practice test.
4. Review your answers to the test paying particular attention to the wrong answers.
5. Try to understand the concept behind the question you got wrong.Although the questions on the SAT change from test to test, the concepts stay the same. If you can understand the concepts that you got wrong, you are much more likely to correctly answer those types of questions on the next test.
6. Repeat steps 3 to 5 several times.
My experience has been that this approach to preparing for the SAT is often more successful than standard test prep. If you do need help understanding some of these concepts, then hiring a tutor may be appropriate to address the specific concepts that you are struggling with.