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 such as the Khan Academy. This might also include studying an audio/visual course on the topic from your library or from The Great Courses Company (when on sale). 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 and use review books or apps.
Develop and use flashcards from a site such as Quizlet and possibly a flashcard app for that topic.
Use study guides such as SparkNotes and review articles such as the Sunday Book Review from the New York Times.
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.