Temporal Balance. . .

Keep your cool during the paper. If you feel you are getting flustered, close your eyes for some time to regain your composure and concentration.

Keep yourself in a cheerful and positive mood.
If you feel tension rising during the paper, take deep breaths to release tension.
Do not get demotivated or overexcited on seeing the question paper. Keep your balance. Demotivation will make even easy questions appear tough and overconfidence will make you commit silly mistakes.
Do not get depressed by tough questions. Instead remind yourself that these questions are tough for every aspirant.
Do not get emotionally attached to any question. It is not necessary that you will be able to solve a question from topics you have prepared well.
Do not worry about the minimum cut-off marks for JEE. No one knows about the cut-off for sure, but no one has ever been rejected on the basis of any subject-wise minimum cut-off.
Do not argue with anyone about anything before the test.
Have faith in God and in yourself. Whatever happens to you will be for the best.

Keep depressing thoughts away
First of all, realize that you are not the only one facing anxiety. Each IIT-JEE aspirant is facing the same dilemma.
Also realize that if you have studied even 60% of JEE syllabus properly, you need not harbour negative thoughts.Whenever you feel tensed, start a countdown from 50 to 1. It will help you release tension.
Light stretching exercise also helps reduce pent-up tension.
If you are feeling tensed, do not suffer alone. The company of friends and light chit-chat will help you divert your mind from negative thoughts.