Now-a-days, you can’t differentiate much between general studies paper of prelims and of mains except from the fact that in mains you have to write answers while in prelims you have to select one. So, you need to prepare a strategy for prelims GS as rigorous as you do for mains. So, a few things must be kept in mind for preparing for GS in prelims.
The first thing is that you must go for in-depth analysis of whatever you study just like you do while preparing for mains. Basics must be clear and sound for the prelims. So, always go for NCERT books before jumping to any other renowned books or guides.
Try to adopt reading habits so that you don’t waste too much time reading lengthy questions. Read newspapers exhaustibly and also go for some current affairs magazines, anyone that you are comfortable with along with Yojana and Kurukshetra.
While reading current affairs, keep your eyes and ears open so that you are able to correlate the recent developments with respective topics of the core subjects. Also try to go deep into the history of all current issues of chronic nature e.g. Syria crisis or Israel-Palestine problem or Indo-Pakistan relation or for that matter any news from RTI Act.
Don’t ignore any of the core subjects simply because in past 2-3 years no questions have been asked from them. UPSC is so unpredictable that it is possible that you might get lots of questions from the topics or subjects that you have ignored.
Keep an eye for various reports issued by various departments and ministries of India or by important global organizations like WTO, UNCTAD, UNDP, UNICEF, UNHCR, UNEP, WEF, World Bank, etc.
Try to indulge in surfing as Wikipedia has emerged as one of the most important companion for IAS aspirants.
Practise as many questions as possible and practise them in examination conditions. You can do this at home or you can join some test series for the matter. In this regard, CIA test series can come handy which not only evaluates the answers, it also conducts detailed discussions where you can get your queries answered by experts. Also, all India ranking gives you an idea of where you stand in the race and you can accordingly adjust your efforts.