Short answer: My Strategy in three words Read, Revise, Write.
Read and practice as many MCQs as you can.
1. First you should pick the subject which you think is easier than the rest, I chose Geography ( being my optional). I'll finish off the topics and after each topic I used to solve as many MCQs as I could. I kept a target of minimum 10 papers of 100 questions (i.e 1000 questions) for each subject. I used to hunt for question banks and solve em.
2. After solving, say, 50 MCQs I'll check the answers and go back to the books and notes to clarify the doubts.
3. Do this for each subject with preferences in following sequence Environment, Polity, Geography, History, Economy.
4. Once you're done with this, pick up newspaper notes and revise thoroughly. These days current affairs form a considerable part of questions.
5. Lastly if you have time, pick up ancient History and Art and Culture.
6. Once you are thorough, try solving simulation test series, previous year question papers ( some questions do repeat).
You are ready to take on the Prelim now.
Optional: I'll suggest to go for classes in case you are not opting for your graduation subject.
1. Finish off your classes (takes around 4 months). Take down notes in class and summarise them everyday in short. I used to prepare synopsis EVERYDAY without fail.
2. Revise the material every weekend and try attempting questions at least think of answers if you have no time to write.
3. Once you finish the coaching revise the summaries and make additions from books if required.
5. Start practicing Mock tests after you are through the points 1 to 4. With each passing test your material will have additions, your answers will start becoming crisp and start making sense.
Strategy for GS: Now this is close to my heart. I was one of the highest scorers with 416 marks in GS in 2015.
Again "read less, revise more."
Your goal should be to reduce your material to as minimum as possible. My full GS material would not be more than even 40 A4 sheets.
1. First I used to read the syllabus before starting studying for the subject. I literally mugged up the whole syllabus ( it helped me in framing answers when I was short of points).
3. Second reading: read the underlined part and make separate notes for each mini topic of the syllabus (eg. One dedicated page for Terrorism in GS 3).
4. Third reading: Before attempting any mock test read the relevant material in your exhaustive notes and only then write the paper.
6. Fifth reading: once your paper is checked, read all the answers written by you and think how it could be better.
7. Sixth reading: read the whole notes in 2-3 days and revise in half day and same day write the full length paper. And revise step 5 & 6.
Again: continuous revision is the key.
I used to revise my GS 3 paper ( the so called longest paper) in 2 hours thoroughly.
Now you are ready to take on Mains.
Here's the sample of my GS notes.
PS: This is my strategy. Which needs selective reading and hell of revising. But works for me.