Based on my own experience that I need to see a given subject thrice before it settles in my head and having seen similar response in my students, I recommend the following for optimal efficiency:
- When you walk into a lecture be sufficiently aware of the subject that you could write (or have written) a short outline of the lecture. This means to read and attempt to understand assigned material . The lecture is then your second exposure. An hour spent prior to the lecture is a very good use of time.
- Take notes only on materials you think you will need to review again or you know is not in the textbook/notes...otherwise pay attention to the lecture and absorb the big picture!
- Do relevant assigned problems as soon as possible. Avail yourself of office hours etc. if you cannot do the exercises or have further questions.
For increasing efficiency, prereading of the material (i.e. understanding which are the difficult bits for you) is by far the most important. Then you can eliminate those bits expeditiously.
And of course you must do the questions to truly flesh out and solidify your understanding.