Do a survey of a course before you take it, such as reading a general history on the topic or several Wikipedia articles on the topic. This may include studying one of the many online university courses or other Top 20 Online resources as described above. This might also include studying an audio/visual course on the topic from your library. These surveys give you a scaffolding in which to put particular facts.
Get off to a good start also by reading part of the textbook or the reading list before starting the course.
For AP Courses, review the course descriptions at www.apcentral.collegeboard.com.
Develop and use flashcards from a site such as Quizlet, Memrise, or Anki.
Use study guides such as FreeBookNotes and SparkNotes and search book review articles such those in the New York Times Sunday Book Review.
Seek out a mentor or study group (without plagiarism) and ask questions.
Like Clint Eastwood in a gunfight, you want an edge. You want the sun at your back.
Justice with Michael Sendal has been the most popular course at Harvard. All the video lectures are online at http://www.justiceharvard.org/watch/. View the first lecture to get a good idea of what the college experience is like.