If You are preparing for the Examination, The first Step or the Mandatory Step is to analyze the Previous Year Question Papers. The Reason behind my suggestion is that it would help to understand the Methodology and to know What UPSC wants. The clear analysis indicates that atleast 5-15 questions are repeated for the Prelims Every year either form as it previous year question or in its changed form. So please do concentrate the Previous Year Papers. I have Posted the link of the Previous Year Papers from the UPSC Website.