ChatGPT custom instructions dramatically improve output quality when configured properly. Two-field structure: "About you" (background ChatGPT should know) and "How should ChatGPT respond" (output preferences). Power user configurations: Technical professional config - About: "I am a senior software engineer specializing in distributed systems. I work primarily in Python and Go. I prefer implementation-focused answers with code examples." Response: "Be concise and technical. Lead with code, then explanation. Use markdown with syntax highlighting. Flag edge cases and production concerns. Suggest related considerations I might have missed." Content creator config - About: "I run a B2B SaaS marketing blog targeting CTOs and VP Engineering." Response: "Match technical depth of our audience. Focus on practical ROI. Avoid vendor fluff. Structure posts with headers. Include specific examples and numbers." Memory management: ChatGPT automatically builds memory - review and edit in Settings > Personalization > Manage Memory to remove incorrect or outdated entries. Add explicit memories: "Remember that I prefer metric system" or "Remember that I work on enterprise software, not consumer apps." GPT-4o Memory combines custom instructions + memory for highly personalized interactions over time.