NCERT books
Do not think for a second that NCERT books will not suffice. In any circumstances, if you want to get a good percentage, you have to study from these books. Maximum of the exam material comes from NCERT books so you can understand its importance.
Make a schedule from the beginning
The main issue with most of the students is that they do not schedule their subjects in the beginning. Most of the students start preparing for final exams only after December which is wrong.
Sleep
Minimum 8-hour sleep and healthy food are very important for the fully functional brain. Do not live on junk food and prefer to take as many nutrients as possible as per your age requirement.
Formulae chart
Make a small chart of all the important formulae and always keep it with you. Read them every once in a while during the day.
Practice
Get old exam papers, use special mock tests etc. to practice your calculation skills. Check for the long questions in the old papers and practice on them as they carry maximum marks.