Work on vocabulary - there are quizlets and other online wordlists which can help. I am a strong believer in old fashioned flashcards as well. There is something about writing the words and definitions by hand that helps make them stick. Go through your practice tests and write down every word you don't know, both on the questions you missed and on the questions you got right. Review your flashcards every day.
Use Khan Academy's SAT study section, particularly for math. SchoolYourself and TheArtofProblemSolving are also good for learning math.
Take practice tests every weekend. Be sure the practice test conditions match real life test conditions. Time the tests and take them on paper, not on the computer. Go through your results and make a study plan for the coming week. Focus on your weakest areas.
If you don't understand why you got something wrong, find someone that can help you understand. There are a number of online "helpers" that will answer questions. Here's one such resource: http://www.erikthered.com/tutor/practice.html