Design Entrance Exam

Coaching yourself will be the best. You will get plenty of platform where you will come across like minded people who are preparing for NID. If you lack self discipline and want an external source there are plenty of Insitutes. I liked this answer by Joseph Tsukka Is Bhanwar Rathore Design Studio in Ahmedabad good for NID and NATA?
I am copying pasting my answer to similar question on CEED and NID coaching institute asked by someone else, which may help you if you are looking for coaching classes:
Bhardwaj Classes http://bhardwajclasses.com/contact/
Many of my Delhi NID friends are from here.
BRDS, Main Branch Ahmedabad
https://www.youtube.com/watch?v=. this is promotional video of BRDS and their Students.
Not getting into details of why and how. just wanted to add I have recieved negative reviews about its owner, some says he got attitude problem due to its monopoly in the coaching market in that region. Though he is good with advertisement and sponsoring. You can find his adv first if you google for NID. It claims to give 60-70% students of NID are from their institute which is also true to some extent.
Their youtube channel https://www.youtube.com/channel/.
Silica Mainly in Mumbai
Silica is Popular in Mumbai and Pune region… some of my senior and junior have thought there. I am just putting link from the webpage for contact which is good enough http://www.silica.co.in/contact.
There you tube page https://www.youtube.com/channel/.
My senior who is host of art attack is also associated with it https://www.youtube.com/watch?v=.
my junior https://www.youtube.com/watch?v=.
other video https://www.youtube.com/watch?v=.
AFD India, Banglore, Chennai region and Delhi
http://www.afdindia.com/contact-.
You tube https://www.youtube.com/channel/.
They are main people julie@afdindia.com vinny@afdindia.com
If you need more info just let me know. there are other institute as well. which is doing good like in Bangal and Kerala where a majority of students come from. I would personally suggest you to prepare yourself.