To prepare for the ethics paper in UPSC Mains, candidates should focus on understanding ethical theories, moral philosophy, and their application in governance. They should review case studies that highlight ethical dilemmas faced by public servants and analyze the significance of integrity and transparency. Engaging in group discussions and practicing answer writing can refine critical thinking and articulation. Additionally, candidates can refer to resources like the Lexicon for Ethics or previous years' question papers to gain better insights and prepare effectively.