A good essay in the UPSC Mains exam should have a clear structure, including an introduction, body, and conclusion. It is essential to present a coherent argument and provide relevant examples that support the central theme. Grammar and syntax should be accurate, ensuring the essay is comprehensible and engaging. Using quotations or references can enhance credibility, while a personal touch or unique perspective can make the content stand out. Finally, staying within the word limit is crucial to address all parts of the prompt adequately.