Practice, practice, practice. At the high school I went to, we started taking practice exams starting freshman year. Math teachers would give a section of the math test probably once a week (not the entire test, of course, but maybe 20-30 questions at a time). Once you get a feel for the style of the test and the kinds of questions that will be asked, it takes a lot of the mystery out of it. Once you know what areas you have trouble with - you know what to study. The same goes for the language arts sections of the test. You can buy practice tests at any bookstore, or online. There's really no more to it than that. Take practice tests ALL the time, study the parts you get wrong, repeat.
(By the way, my high school was in the 90th percentile nationwide in terms of average SAT scores - so the practice, practice, practice method really does work! =)