Firstly, we should be completely thorough with the syllabus. Syllabus of JEE Main is based on N.C.E.R.T but in Mathematics, we need to be a bit extra. We need to cover Arihant and other books for some topics, else RD Sharma and NCERT  do all of it.

Next thing is staying updated about the pattern of the exam. There is a fixed 360 marks paper (120 marks for each of the three subjects; Physics, Chemistry, Mathematics) and the pattern remains same most of the times; but since last two years, the pattern has been changing because the exam governing body was handed over to National Testing Agency (NTA)  which declared the exam to be taken twice a year; January and April; and 4 days; in 4 shifts per day.

Next is we need to be aware of the marking scheme : it is +4 for a correct answer, -1 for a wrong and 0 for the question left unanswered. But, you never know! They may try something new for the respective year.

Next is, referring to the right reference books. There are many publishers, authors of the same subject. It can be possible that despite of reading hundreds  of books, you might not be thorough with the subject whereas on the other hand only 1 right reference book can clear your concepts. The reference books that I referred and would recommend anyone to refer for the exam are as :

1. Physics : H.C Verma, NCERT Textbooks

(Also, I.E Irodov can be considere an ideal book but it contains some advanced-level questions which are not required for JEE Mains. )

2. Chemistry : NCERT Textbook (most importantly). Other than that, Himanshu Pandey (Organic Chemistry), O.P Tandon(for physical chemistry) and again, NCERT for Inorganic Chemistry

3. Mathematics : R.D. Shamra, Arihant Maths for JEE Mains, and NCERT

Tip : You can see that how important it is to be thorough with the NCERT textbooks. Also,  2018 topper Kalpit Veerwal , who scored 360/360 in JEE Mains suggested in one of his videos that NCERT helped him throughout his preparation.

Last but not the least, practice mock papers uploaded and prepared by various sites. Also, the most helpful papers are the ones of the previous years. Solving previous year papers help in analysing about the weightage of the marks, topic wise. Also, it helps in understanding the pattern and gives a hands-on practice of coping up with speed and improves the understanding about how questions can be asked from exam point of view. Also, many of the times questions are repeated in the paper so it adds as an extra advantage.

Experts give this tip that there are some topics which take a lot of time and can also be our weak point, so we should not waste our time on those topics when the time for preparation shortens.

All the best for exams!

Hope this helps :)

Feel free to ask any other queries related to that.