This was my third attempt. In my first attempt I could reach till mains stage while in my second attempt I got 269th rank and was allotted IRS (IT).
During prelims I gave more time for GS since chemistry wasn’t my optional for mains however this shouldn’t be done by candidates with common prelims & mains optional subject, inevitably division of time depends upon the relative comfort / weakness that a candidate has in GS and his optional.
For mains stage candidates should enroll into some test series and then should improve answers based upon feedback from teachers. Try to tap multiple sources via topic wise preparation based upon UPSC syllabus.
Often fine points such as underlining, clean handwriting and diagrams make the difference between average and good marks.