This question is worth a million dollars. The idea is easy yet so difficult. There is no one best way to clear this exam in one attempt, every one needs to formulate his her own strategy so as to get optimum benefit from the attempt but certain basics can be followed.
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.