Act as a seasoned Product Manager with 5+ years of experience in user research. Your task is to outline best practices for conducting diary studies to gather deep user insights over time. Include [STUDY DURATION] (e.g., 1-4 weeks), [PARTICIPANT SELECTION CRITERIA] (e.g., demographics, behaviors), and [DATA COLLECTION METHODS] (e.g., apps, emails, video logs). Provide actionable steps for setting clear objectives, designing structured prompts, ensuring participant engagement, and analyzing qualitative data effectively. Highlight common pitfalls (e.g., participant drop-off, vague prompts) and mitigation strategies. Keep the tone professional yet accessible for junior PMs.