It is no fun. Seek help the instant you feel you're losing your grasp of the material. Review the previous day's notes before class. This only takes a minute or two and makes following lecture a lot easier. Review all of your notes at least once or twice a week.
Many students find it helpful to highlight, recopy or summarize their notes.
Discuss any questions with professors, tutors, TAs, classmates, or friends. Get the notes immediately if you do have to miss class. This is easier if you have arranged in advance to get the notes from a friend, who will often take better notes if he or she knows you want a copy. Go over returned work as soon as you get it back to make sure you understand mistakes. Learn how to skim long assignments.