Why wait until the last minute to spend 15 hours cramming when it can be spread out over time?
Create a study schedule. In previous semesters I dedicated one day of each week per class. For example, on Mondays I would do any work that was due the next day (if it wasn't already complete) and then study for Econ. On Tuesdays, I would work on Statistics. Et cetera, et cetera. This semester, I am dedicating a little bit (30ish minutes) of every day for each class.
Re-write notes. This semester, I have dedicated Saturday nights to re-writing my notes. I don't go out much, so Saturdays work for me. Choose a day that works best for you. By re-writing your notes, you reinforce the items covered in class. Bonus points if you take your notes by hand. Be sure to have both the powerpoint slides and the book open for your reference!
Form a weekly study group. Last semester, my friends and I got together once a week and did our homework together. Afterwards we went to eat or grab ice cream. Studying with people in your classes is encouraged in case you have questions but it is not required. Literally all of my friends are science majors and I'm a business major (go figure). It's sometimes easier to focus when you know you have to do it alone.
Do your work earlier. When time permits, do your pre-reading ahead of schedule. There is no harm in writing notes ahead of schedule. In fact, it may even help you understand previous chapters.
Actually do the homework. Even when homework is not assigned, it is important to practice. If you spend the time during the unit, you will have less to try to learn before the test. It's a pain but "no pain, no gain."
Don't wait until the last minute to go to ask questions. How many times do I have to say go to office hours or ask questions (see entire post dedicated to office hours here)? Do not think that it will get easier with time. If you do not ask questions (or someone else does not ask your question) then it will be harder to understand connecting topics.
If you don't understand a topic, it will not get easier until you take action.
Don't expect office hours to be empty the day before a test. Everyone will be in the professor or TA's office asking last minute questions. Go for quick questions or just to hear others but do not expect to get a very high grade from simply studying the night before and going to office hours (unless this has worked for you for years, then who am I to stop you).
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Tell me in the comments how far in advance do you start studying for exams?