For almost all of my SAT/PSAT students, I use the 2nd edition of "The Official Study Guide to the SAT" (see above), which is by The College Board, the maker of the SAT and PSAT. The book is around $20 on Amazon. It has 10 practice tests, some of which are actual SAT tests. I have used other books, and the problem with almost all of them is that the questions, as a whole, are not exactly like the real SAT questions. Indeed, there is a marked and obvious difference.
If a student is particularly motivated, I suggest studying vocabulary books and flashcards. See my "SAT Reading List" page.
Normally, I assign a practice test to be done as homework. Obviously, at the beginning the student should not time these sections. (When you learn how to play tennis, do you first learn how to execute shots, often in slow motion, or do you do everything as quickly as possible?) Worrying about time shouldn't become an issue until two weeks or so before the test. The first step should be learning how to correctly answer the questions, not learning how to shave off seconds.
The student should star or otherwise mark each question that seems difficult. I prefer that students do not guess when doing homework, for obvious reasons. (If a student guesses, then there will exist questions the student got right only by chance. If the student does not learn how to do these types of questions, then the student will probably miss most of the questions of these types on the test since the student would have to revert to guessing again.)
Either before, during, or after answering the questions, the student should look up every notable vocabulary word on the test and neatly write down the word with its definition in the book. There is plenty of room to do this. The student should try to memorize these words and their definitions.
After the student is finished with the test, the student should grade it carefully. The answers are at the end of each test.
When I tutor, the student and I will go question by question through all of the questions that were missed or starred. I use a grammar book by Diana Hacker, along with other resources, for the writing and reading sections. I go over what to do and what not to do on the essay, taking apart various essays. For the math I have to give mini lessons on various topics, and I love to show students different ways of doing the same SAT Math problem. That's one reason SAT Math problems are so different from regular math class problems.
I go through this cycle over and over. With some students, I throw in some non-SAT questions, but for the most part, I stick with The College Board book.
The first thing you should look for in an SAT tutor is the ability get the right answers by logically solving the questions. As obvious as that advice should be, many people get tutors who cannot even figure out the questions.
While I do teach some test-taking strategies, many of which I keep secret, some of the "test-taking tricks" that tutoring companies teach are utter nonsense. One I often hear is to not put the same letter for four questions in a row. So if you answered "B" for three questions in a row, then they claim you should not answer "B" for the next question, even if the answer seems to be "B." As stupid as this sounds, this is what some tutoring outfits will tell you (after they take your money). Of course, you could have answered one of those three in a row questions incorrectly and the fourth one is in fact "B."
Other baloney I hear is the teaching of grammar rules that are not tested or grammar "rules" which aren't even rules. If someone tells you not to split an infinitive and not to start a sentence with "and," "because," or "hopefully," then you should look elsewhere for advice on English and the SAT.
Finally, I'd like to disabuse some of you of something that rankles me to no end. I was a teacher, and I can assure you that not all teachers are created equal. Just as all doctors or auto mechanics are not the same, so all teachers are not the same. Although I had some good teachers, if I had a nickel for every falsehood I was told by a teacher, then I could have retired right after graduating college.