1. Study all ncerts it is a must for a good base.
2. Refer to basic books like bipan chandras, laxmikant etc.
3. Make proper notes as the are the Savior at the time of revision.
4. Join a good test series- it is really important as it gives you an opportunity to assess yourself before upsc does.
5. Do not accumulate material and books from multiple authors from the same subject.
6. Be very selective in what you study.
7. Be confident and keep yourself consistently motivated.
In short if you do the basics right your will be able to clear the exam in the first attempt itself.