I have appeared for many national/international exams held till +2 level, and have fared decently. I feel that the following points highlight my observations about the must have qualities while preparing:

Materialistic qualities:

1. Previous papers practice, both chapterwise, and full length paper attempting.
2. Completing homework given by teachers sincerely and on time.
3. Giving tests on a regular basis, and analyzing your mistakes after every test. Find out how many marks could you have scored more without any extra preparation(i.e. marks lost which were not supposed to be lost).
4. Doubt clearance.
5. Completing syllabus before time.
6. Revision on a regular basis.

Not-so-materialistic

1. Have a good and fixed time for sleep(7 hours minimum).
2. Go for a walk/light sport regularly to stay fit and be health...