Choosing an optional subject for the UPSC mains examination involves assessing one's interests, background knowledge, and the syllabus of each subject. Aspirants should consider subjects they are passionate about and can relate to their general studies. It is also beneficial to examine previous years’ question papers for the subject and consult with current aspirants and mentors. Ultimately, aligning the optional subject with personal strengths and future aspirations will enhance motivation and performance in the examination.