Here are my 2 cents:
- Do not prepare separately for board exams. JEE preparation is a superset of almost all related exams and if you are thorough with your JEE preparation, everything else will fall into place.
- Learn to write. Those who have been preparing for JEE tend to develop a habit of ticking rather than writing. This of course will not work when you are talking about CBSE/ICSE/other boards. They want proper formulae and steps along with detailed calculation. Make sure to do that. It is recommended that you follow the sample solutions from various textbooks. For instance, there are several books in the market that contain many solved examples. Go through them - that should give you a good idea of what the examiner expects from you. Note that I did not use any such textbook for CBSE. I used only and only NCERT. I did so because I never faced any issue in writing detailed answers because it was built in my DNA since my class 10th board exams.
- Do not leave anything. I will tell you a secret - there was a question about plane polarized light in CBSE 2013. This was one of those topics that was present in JEE Main/CBSE syllabus, but not in JEE Advanced syllabus and so, I did not prepare it quite well. However, in the exam, I wrote something. I did not leave the answer and eventually I got marks for that. This was pure luck though. I still do not know how I got 100/100. At the same time, be prepared to lose marks unexpectedly as well. My Maths paper was 100% correct, but I ended up with 95/100. In the end it did not matter to me because essentially my Maths and Physics marks got swapped :P
- Past year papers. These are very important and will give you a good idea of what type of questions to expect. Again, I did not solve any past year papers for Physics, but I remember doing this during my class 10th board exams.
Overall, I would like to say that 100/100 is more of pure luck. Scoring 95/100 is more like hard work, but above that, things are not in your hand. You might end up losing marks where you deserve and on the other hand, you might get free marks at other places.