There is already enough number of tips and fundaes being discussed on various forums to how to write better answers in Mains. Well, I would spare these repetitions and wish to share just one perspective which I got hold of during my own preparations.
Take every question being asked in the paper as a question posed to you by a media person or as simple as a child while you are interacting with an open assembly in a school. So get under the skin of a bureaucrat and try to simulate how would you have answered in the given situation with right tenor and overall temperament. For ex:
GS Paper 2 Q.No. 10 ‘In the Indian governance system, the role of non-state actors has been only marginal.’ Critically examine this statement. In the given case, imagine if same would have been asked by a reporter to you (Law enforcing officer) How would you have defended or supported the given statement with relevant examples, precedents or antagonistic cases.
GS Paper 2 Q.No. 8 ‘ What is a quasi judicial body? Explain with the help of concrete examples’ If a senior secondary child asks this question, I would focus on what exact powers are attributed to it in relation to a common legal authority. For ex summoning, issuing orders, investigation, seizures, statements etc. And I would try to highlight each of these powers by taking relevant examples of existing quasi judicial bodies like FC, EC, CAG, CVC etc.
GS Paper 3 Q.No. 7 ‘ What is water use efficiency? Describe the role of micro irrigation in increasing the water use efficiency’ In this case I would first define the term water use efficiency. Then would emphasize upon current statistics of same within the country (and if possible of other countries like Israel in this case). Then would just mention various reasons of such low efficiency in the country. And in last part how micro irrigation techniques can help (again relevant examples of other geographical locations or within the country statistics). More importantly it can also be discussed why high water use efficiency has become the need of hour in the country given the inter-state water disputes, hygiene quotients, agriculture productivity etc
This approach might actually not help in building your content(matter) but can at-least enrich your experience( i.e manner of answer writing) to how to take a bow and deliver a most relevant, succinct and compact answer to your audience.
Also, in coming years it won’t be necessary that as an officer you would always be asked only from a prescribed list of readings or would always get a time to rehearse and practice beforehand. Some statements, some answers would be given spontaneously with adequate use of common sense and already accumulated knowledge of the issue at hand. Similar is the case in UPSC mains too, where there will always be few questions which we might not know accurately but with legitimate assumptions and balanced use of language we can write about them too.
And in those cases where we might not have faintest of idea, we can leave it as we would have done in front of real audience instead of saying something really stupid.