In my opinion, the best way to study for SAT II's if you're still in an American high school is to take the respective AP class. That way, if you're serious and study hard all year in school for the AP test, you'll definitely be prepared for the SAT II in the same subject. In fact, SAT IIs are offered in May and June around the same time as AP tests, so all you need to do is sign up for those dates and you'll be good. There will be little studying to do outside of your normal class studying or AP test review because all that will have forced you to learn the necessary material. That's all I did for my SAT IIs and I easily got over 700 on the 3 or 4 that I took.
If you're not into the whole AP thing, you can always buy one of those SAT II review books from publishers such as the Princeton Review, Kaplan, Cliff Notes, Spark Notes, Baron's, and many others. If you decide to go this route, you should look up reviews on Amazon or other sites to see which is best for each subject to get the most for your money. Generally speaking, they're all pretty good if you take out the time to read through them, and most importantly, take the practice tests they provide for you. The best way to learn the material is to sit down and practice taking these practice tests, as they simulate questions that might come up on your actual SAT II and because they help familiarize you with the test's format.
If you have any questions on the SAT II Subject Tests themselves, let me know. I took Chemistry and Physics last year and I still remember a lot about the tests. I'm debating whether or not I should take Biology this year as a precaution in case I do end up applying to some private Medical Colleges.