First thing you must know about Joint Entrance Examination is that it is a problem solving test rather than just rote learning. JEE mainly focuses on how critically you might think and how efficiently you put all those multiple concepts together in each question.
So, focus on improving your thinking abilities rather than just mugging up the concepts.
The key to succeed in JEE is not to memorize your syllabus, which almost never works. Rather, it is about solving problems, a lot of problems.
The more, the merrier! The harder, the better!
Tackling past exam series is one way to go, and to get better at it, preferably under simulated exam conditions. At its core, preparing for JEE entails and necessitates a thorough understanding of each and every component you study about, and their inter-relationship in the formulas and equations.
You also need to spend an adequate amount of time to understand and work on the concepts that you study. The best way to understand your concepts is to first build your basics than attempting harder problems in the first go. The next step is to put your acquired knowledge into problem solving, covering a countless variety of new problems from competitive books, or taken out from your textbooks, all on your own.
At the other end, solving problems must not be the only thing that you care about. Performance analysis is a must, go through your error patterns and immediately work on eradicating your weaknesses.
What you study is embedded in the nature so it is relatable. Like many things in nature, the facts studied in Physics, Chemistry and Mathematics are discovered, not invented and their properties are still being discovered.