First of all, I'm just here to share my experience with the SAT I. Honestly, I'm not a really "bright" kid who can memorize everything a teacher says; I have to study quite awhile for tests and such. So if you're like me, perhaps and most likely, this will help. If you're not, and is one who learns quite easily, then you might reach your goal easier with my advice. Take it with a grain of salt. I'm not here to brag; You may disagree with my ways, and that's fine.
My 170s in PSAT (forget exact score) to 2310 SAT I (took it 2 times, superscored) took approximately one year, including a summer. I got accepted to two of HYPS.
1) Memorize 50 words a day from the Barron's Word List of 3000 words (I realize a lot of you guys might disagree with this, but it worked for me; on BOTH tests, all the words I saw, I recognized, and even though I might not have grasped the exact meanings, I got enough to eventually decipher the correct answer confidently.
2) Read good books and newspapers (NY Times, Economist, etc)
3) Take all practice tests from Blue Book, Barron's, and Princeton Review. This probably helped the most. Practice makes perfect. REVIEW your mistakes after EVERY test you take. This builds confidence too.
4) The Grammar Section in Barron's is superb. Read it.
5) I never had a problem with math, but do Barron's if you do have a problem with it. It has hard problems, but that's better in preparing you for the real thing.
6) For Critical Reading, I didn't choose which passages to read or w.e. first. I did it question by question. I read the passages in its entirety then answered the questions. Trust me, you have enough time. When you review the questions you missed in CR, know why it's wrong. You'll slowly develop a sense for choosing the correct answer. This sounds weird, but it's true. The correct answer slowly begins to just pop at you once you do a lot of practice tests.
7) Essay: THREE examples if you're not a great writer. Preferably historical. I didn't outline (got a 12), but just started writing. Write what you CARE about; that's when you shine and get high marks.