Going to class and getting your schoolwork done aren't the only things you should be doing while in college. What you do in these few short years will affect at least the first ten years of your work life, if not all of your working years.
Why? Because of a little thing called "networking"--simply put, building relationships with others in your field. If you don't do at least a little of this, you will suffer in your career. Please learn from my (very expensive) fail.
Network with Professors
Think of the professors in your major as professionals in your future field, because they are. You need rapport with them if you're going to have good references coming out of college, and the following tips will help you obtain that:
Attend every class unless physically unable, to show your dedication and dependability.
Talk to your professors after class; ask questions, comment on the lesson, or just thank them for their time and effort.
Write emails to your professors in a clear, concise, and professional tone.
Advise your professor ahead of time by email if you will have to miss their class, and ask what material will be covered.
Ask your professors about any internships, calls for papers, conferences, or anything else in professional development.
Network with Peers
Your peers might be asking you for study notes (or vice versa) now, but one day you could be asking them for a job reference (or vice versa). Don't ignore the unique opportunity you have to make good professional friends!
Make at least one academically-minded friend in every class, and keep in contact with them regularly even after the class is over.
Participate in academic fraternities and clubs in your major (and even outside it, if possible).
Participate in online groups of people in your major, even after you graduate!
Seek out opportunities to mentor or be mentored by peers.
Involve yourself in community groups outside the college sphere.