Common mistakes which you should avoid while preparing for JEE exam is mostly students thing you should revise the syllabus at the end when 1 month is left. You should daily revise your syllabus. If you want you can set your time table in which you can allocate a time for your revision according to your strength and weakness.

You should always clear your doubt. Don't hesitate to ask your doubts. 

Don't try to solve the problems which are of high level then JEE Main. Prepare according to the syllabus and try to clear your concepts to score good marks. 

All the Best!!