Use a planner or other organization system. I've had my day-planner for years and cannot go anywhere without it. Others are that same way with their personal digital assistants.
Stay current with due dates/course calendars. It's not enough to have a system -- you have to use it! So once you have some sort of system, get in the habit of using it (and it will soon become second nature).
Keep homework, tests, and class papers in central location. Don't just throw old homework assignments or tests in the back of your car or the floor of your dorm room. You'll need these for studying for future tests, for meeting with your professor to discuss them, and for figuring your grade in the class... so, keep all your class materials in a central location.