Pick a recorder. You can use a digital recorder or an older one that uses tapes. The advantages of a digital recorder are that you can easily scroll through the lecture, easily delete anything you don't need and easily transfer the lecture to your computer.
Take your recorder to class. You should keep the recorder out on your desk during the lecture, so that the recording is not muffled. Sitting as close as possible to the front of the room will also help you get a better recording.
Start the recording when the lecture begins by pressing the "record" button. Usually a red circle also means "record." If you are using a tape, make sure that it is not in the middle of the previous lecture; otherwise you will record over it. Since digital recorders automatically start a new recording, this will not be a problem. Pause the recording when needed and remember to stop recording when the lecture is over.
Write your initial notes during class, even though you are recording. This way you are not in a bind if your recording doesn't turn out well or if it is accidentlaly deleted. You should never rely on your recording to jot down your initial notes.
Compare your initial notes to the recording after you get home. Turn on the recording and listen to the lecture, while reading the notes you wrote. Pause the recording to take down any notes that you missed or to add additional information. This way you won't miss parts of the lecture.
Pay close attention to the recording for clues regarding the most important points in the lecture. If the instructor repeats something more than once, make a note of it, as it is likely important. If he uses an emphasizing tone, make a note of that as well. This will help when reviewing your notes for a test.