Use Calendars Effectively

I used two calendars in college, one to much better use than the other 

Google Calendar
Get a calendar set up in Google Calendar for just your classes.  Put your class schedule in it for the whole semester as soon as you get it set.  Once you get your syllabi, go through them all and put in EVERY MAJOR EVENT.  This means quizzes, exams, midterms, papers, finals, and anything else worth noting.  Update these things if they get moved.  You will never forget about an assignment ever again. 
Plan your days.  Your calendar shows you when you have class.  Put meetings, events, work, etc in their own colors.  This is a great way to find open spaces of time and minimize friction.
Your calendar on Google is available anywhere and can be shared with others.  Use it. 

Week By Week
I used a week by week for my first two years, then stopped carrying one.  Some people find them essential to planning out their weeks and putting in important dates.  Your important dates are in your Google Calendar.  The only real reason to use one of these is to write things down that the teacher assigns that aren't already in your Google Calendar.  When you get to a computer, you can then transfer these items to Google.  If you need a week by week for these shorter-term items or because you are apt to forget things before you get to a computer, by all means, use a week by week.  If not, don't.  Give it a try for a while and see if it's useful for you.

I'm sure there are other useful calendaring options out there (I went for a plain sheet of paper for "don't forget to add this to Google" stuff day-to-day), but these two are my recommendations.  At the very least, learn to use Google Calendar effectively.  It will be a lifesaver for prioritizing your work and optimizing you days.

Marginal Benefits

Marginal benefits and marginal costs are the basics of microeconomics.  Whether you're aware of it or not, every decision you make comes down to the marginal benefit and marginal cost of that decision - that is, the additional benefits and costs to you associated with the decision.

Example.  At what slice do you decide to stop eating pizza?  When the marginal benefits - the amount of pleasure and fullness you get from the next slice, are outweighed by the marginal costs - monetary cost, getting fat, feeling sick to stomach, etc - of that slice.  Often the marginal benefits actually diminish as time goes on.  That is, you get the most benefit out of the first slice, and each additional slice gives you a little less.  We call this concept diminishing marginal returns.

Another example.  When you're kissing someone, when do you decide to stop?  When the benefit you receive - happiness or whatever, also a diminishing return as time goes on - from an additional second of smooching, is outweighed by the cost - in this case, the missed opportunity of whatever else you could be doing instead of smooching.  We call this kind of cost opportunity cost - the benefit of the next best option that you give up when you choose something else.  The opportunity cost of kissing could be going to the gym, or talking, or whatever you would be doing if you weren't kissing.

Anyway.  While you will discover that this concept really does apply to everything you do, it has specific application to study habits, which is the whole point of discussing it here.

When you're studying, every minute you put in studying something has a marginal benefit (additional knowledge of that area) and a marginal cost (whatever you could be studying or doing instead).  The more you study one subject or even topic within a subject, the more you know about it, but also the LESS you gain from each additional minute of study.  The first hour you study one thing is WAY more productive than the 5th.

So, as test day approaches and you are making decisions on what to study, ask yourself "What area will I get the most marginal benefit from studying?"  Assign your next study hour to the area that will net you the most points on the exam as a result.  Reevaluate often.  As you study one area, the benefits of continued diminish.  Reallocate your study time accordingly.

It's also worth pointing out that these principles also tell you when to stop studying.  At some point, the marginal benefits are so small that it's logical for you to be done studying.  When the marginal costs (doing something else - probably more fun) outweigh the gains (another half a point on an already A+ test grade), it's time to move on.