1. **Clear and Concise Documentation:** Well-written documentation explains *what* the API does and *how* to use it effectively.
2. **Versioning:** Versioning manages API evolution; avoid breaking changes. Use semantic versioning. Consider a clear versioning policy.
3. **RESTful Principles:** Adhere to RESTful design principles for consistency and flexibility.
4. **Error Handling & Validation:** Implement robust error handling to provide informative responses. Validate input data.
5. **Security:** Protect against common attacks (e.g., injection, cross-site scripting). Implement authentication and authorization.
6. **Testability:** Design your API to be easily testable (unit tests, integration tests).
7. **Responsiveness:** API responses should be fast and suitable for the client application.
8. **Idempotency (where appropriate):** Ensure operations can be repeated without unintended side effects.