Try each one and see which one works most effectively for you! My personal choice is a combination of the outline method and sentence method, especially when I have a computer and can type my notes, because I can more effectively capture what the professor actually said in class. When I don't have a computer, I go solely outline method, summarizing and organizing the speech/lecture into topics.
Look over class notes no more than 6 hours after the lecture, preferably with a study buddy so that you both can have a more complete set of notes.
If you're really struggling, find out where your school's tutoring center is and set up an appointment. Sometimes professors as well as older students do tutoring on the side!
Relate difficult subject matter to a subject you like and enjoy. For instance, I visualized the plotlines of four Shakespearean plays and used that as a memory tool, to help me remember the visual graphs of functions in College Algebra. (Graph starts high and ends low = Romeo and Juliet; graph starts low and ends low = Hamlet, etc.)
Study differently based on what kind of test you're gearing up for:
Multiple choice test? Make sure you know definitions, vocabulary, facts, and main concepts.
Fill-in-the-blank test? Pay special attention to how the professor phrases main concepts in class, how your textbook phrases ideas, etc.
Essay exam? Focus on a few key concepts that your instructor has discussed heavily during class, and be prepared to go in-depth.
Note: I am a former English Language Arts teacher and thus am making these recommendations based on that experience. I believe the following tips will be applicable for every discipline and major, but go by your professor's/program's recommendations first.
To flesh out a paper, figure out WHY your sources felt right for this paper. What do they tell you that is new, intriguing, or even angering about your subject matter? Discuss each interesting point in a paragraph.
Don't rely on formatting tricks to make your paper longer--professors can catch that easily. Focus on actually writing something of import.
Do not ask someone else to write your paper for you, or use essay-writing websites. Professors will know it is not your work and you might even get kicked out of college for it.
On an argument paper, keep in mind that your professor is going to have to read a ton of these papers. Write something interesting. Write your authentic thoughts on this subject. Introduce your topic thoroughly, and then show why you think the way you do.
When drafting an argument paper, first try writing out a list of thoughts you have about your subject, and then find evidence in text(s) to either back up or refute your position or opinion.
On research essays, dig up the most interesting and credible sources you can find on your topic. Look through old books, new books, old and current newspapers, lots of websites, microfiche film, video or audio interviews, ANYTHING. Find out how current thought differs from traditional thought on this topic.