Hard-work itself isn't, but combine it with efficiency. It is important to actively learn from you mistakes, and making a conscious decision and action to see where you went wrong, how this same mistake won't be repeated in the future. When preparing for an exam, it is highly suggestive to only focus on materials that you do not have good concept of. Do not spend too much time on concepts that you have a good grasp on. Time is very important when preparing for an exam, so use it wisely.
Good study tips:
1) Remove any clutter from the study desk, and only preoccupy with the study materials that you are currently focusing on.
2) Take a 5 minute break every 50 minutes, if you plan on continuously studying for a long period of time.
3) In science subjects DO NOT assume you understand the material because you did the reading. Even though it seems to make sense and seem familiar, be sure to work out challenging problems, and judge your progress from the ability to work the problems correctly.
4) To improve writing skills, attend as many writing workshops near you as time permits.
5) Another good tip for improving one's writing is to show peers and ask them to proofread the paper to catch any problems that you missed. This will also help you become aware of obscurity and vagueness in you writing, as well as improper grammar usage
6) Study groups should be frowned upon, as it is more likely to be detrimental than beneficial.
7) If concepts are not being understood properly, look into private tutoring and that is what may as well differentiate an "A" from a "B" or "B" from a "C."