Have the goal of being the best.
It may sound silly, but if you decide to be first, you will be the first. If you decide to be in the first half of your class, you will be in the last of the first half. Which, you will agree, is not really compatible with the idea we have of the concept of excellence. Do you find the idea of being first in your studies too unreal? So, try to focus on the goal of being first in each of your classes. The end result will be the same, but it is less impressive and it seems much more realistic. Do not set a goal for grades: teachers have different scales and sometimes it will be easier to have 17 in one subject than 12 in another. Do not hesitate, ignore taboos and compare yourself to others.
Know what you are logged on.
At the beginning of the year, your teacher will usually tell you what your average will be - if not, ask for it. Some will score you especially on your final exam, others on a research project, others will make you write essays that will count for 60% of the final grade of your part. Know what your case is and check out here to get more info about all the assignments. This will save you from working for hours and hours on something that ultimately counts for very little in your final score, neglecting the main work.
Learn right after class.
If you learn your lessons a week before the partials, or even a month before, it's for a lost cause. You must make revisions by the evening, and know your course every weekend as if the exam was Monday morning. This will save you from having a very bad last month of classes!
Prepare your HW and your classes.
Answering the questions we ask you before each class, it's nice. You know what's even better? Understand your texts, research and get to know your HW topic at your fingertips. Learn as much as the teacher, answer all questions. One advice I can give you is to find a partner with whom to do a preparation competition; choose a subject in which you will face. Whoever makes the most constructive remarks in class will have the advantage over the other. It's stupid, but it's quite funny (when you start!) And it allows to motivate several, for a stake closer than the final exams. Do not book this with the HWs either; you should know what it is when you are in class, and have already at least a little studied the subject.