When selecting an optional subject in the UPSC Mains exam, candidates should consider several guidelines. First, they should assess their own interest and passion for the subject, as enthusiasm can enhance thorough understanding and motivation. Additionally, candidates should review the syllabus complexity and overlap with the General Studies subjects for more synergistic preparation. Also essential is the availability of resource materials and guidance, as well as understanding previous year trends of mark distributions for different subjects. Ultimately, a well-informed choice aligned with individual strengths and resources is crucial for effective preparation.