The SAT is a gameable test, which means that if you know what to look for you will be able to improve your score. Taking practice tests will allow you to get to know the kinds of questions you will be seeing on the test. The College Board posts practice SATs on their website which are worth taking, as well as publishing a book of ten practice SATs. The College Board is the institution that administers and writes the SAT, so taking their practice tests insures that you will be seeing the same material and types of questions on the real exam. Kaplan and Princeton Review write their own practice tests, which means that it may not be the best representation of what you will see on the actual test. The Edge in College Prep only uses old SATs written by the College Board for our practice tests, so you can be sure that your practice scores are reflective of what an actual one would be.
Secondly, reading about the SAT will help you prepare too. If you look at the types of questions that appear on the Math section you will get an idea of math concepts you should review before taking the test. Algebra and Geometry are the main focus on the test, but the test has certain types of questions they like to ask which might be an unusual format for you.
For the reading and writing, practice tests will help you see what it is exactly that you should be looking for when reading passages or correcting sentences. Brushing up on grammar will definitely help for the writing section, and reading the answer explanations for questions in the CR section will help you understand what it is they are looking for in a question.