 First of all, go through the syllabus and do have substantial knowledge on each word or topic that is written in the syllabus. That will help you know almost all the questions that are asked in the mains examination. Because if you see the syllabus and the previous year question papers, you will notice that many of the questions are directly on the line of the syllabus that is mentioned by UPSC. Secondly, do practice. So the formula has to be revise, write and repeat. If you revise your topics, give the test or write the test and then you reverse engineer. You will be able to add a lot of material to the notes that already exist. So that way you will be able to enrich your answers in a beautiful manner. When you revise things, write them and repeat this strategy over and over again. Then your practice of completing the answers also enhances.