Pick the right field types, add validation (required, min, max, enum, match), use defaults, index the fields you query, use refs for large N, embed small data, keep documents small, use timestamps, use enums for fixed sets, and hash sensitive fields.

Use ref for one-to-many with large N (messages, connections). Use embed for small, tightly coupled data that always loads together (an address inside a user). Embedding large or growing arrays bloats documents; use refs instead.

Pass { timestamps: true } as the second argument to new mongoose.Schema(). Mongoose will set and update both fields automatically. Use them; you will want them later for sorting and debugging.