Please mention which year you are preparing as this year exam is done on 4th November for Mizoram, Nagaland, Andaman and Nicobar Islands and all other states and Union Territories it will be held on 5th November.
General Preparation tips are given below -
Know your Strengths & Weaknesses - You should better know that what all weak areas you have as you will have to work really hard on them to make them all your strength.
Follow NCERT Books - The best books which will be helpful for you would be NCERTs as these are really good to make all concepts strong and clear.
Strengthen Your MAT Section - The MAT section tests your mental ability. Hence, you must give sufficient time for preparing for this par
Work on Your SAT Section - Follow NCERT for the scholastic aptitude section of NTSE. You must also read a good general knowledge book for ‘out of the course’ questions.
Solve Sample Papers - These will be helping you to know all important questions so practice them all properly.
Analyze Your Performance - Better to do online mocks as these are the best source to know and analyse your test performance and online mocks will also make you understand the time management skills during exam along with the actual paper pattern details.
Self-Study is Important - One of the major point which you will have to follow and last but not the least Practice, Practice & More Practice.

Also be confident, do not cram.
Good Luck!!!