I did not discover how to be a 'serious student' until half way through graduate school - which is a bit long!! I used to study like a fiend, and every time - what i studied was almost never on the exam. Instead, a bunch of stuff that i hadn't looked at was on there. Consequently, i did poorly - almost as though i hadn't studied at all.
I finally hit upon the idea of using my lecture notes to cue me onto what the instructor thought was important. I kept track of how long the instructor spent talking about each topic, and how much work they did in talking about it: did they just talk, draw on the board, have prepared hand-outs to give to the students, use view-graphs and an overhead projector. Figuring that instructors don't want to do extra work any more than students do, i figured that the more work, and the 'harder' work that the instructor did, the more important they thought the topic was, i would multiply the time spent by the 'work factor' of what they did.
As an example, say that 'just talking' rated an 'effort' or 'work factor' multiplier of 1 through 3 (depending upon complexity, and that drawing on the board rated 4 thru 6 (again depending upon complexity. Let's suppose that the instructor first talked about topic "A" for 5 minutes (a 'medium complexity' talk). They then drew diagrams on the board (again medium complexity drawing), for 8 minutes. I would multiply the first interval [5], and then multiply the second 'drawing' interval of 8 minutes by a 5 (medium complexity drawing = 40. Adding the two together gives 50. I would do this for each time 'Topic A' was mentioned in the grading period, and total up the numbers. Ranking all the topics in decreasing order, gives a list of the instructor's 'importance' for each of the topics. This is your 'priority list' of topics for the course.
Most tests last about an hour. Estimate how long it would take to solve the average problem. This gives you an estimate of how many problems can be done in the hour. If you estimate that 8 problems can be done in the hour-long duration of the test, make up a list of problems for the first 10 or 12 topics that came out at the top of the instructor's priority list. Include a few extra topics for safety. Make up problems on those topics that include everything that you saw in homework problems or example problems in your textbook or that were discussed in class. Make sure that you can answer or calculate absolutely everything about these topics.
If you do this, you should be well prepared for the exam. Remember that you are always responsible for EVERYTHING presented in lecture, and that just because a problem on a given topic appears on a mid-term exam is NO guarantee that it won't be on the final exam too!! Best of luck!!