The OpenAPI specification is a widely used format for representing RESTful APIs, offering various tooling support to developers. To write an effective OpenAPI specification, it's essential to follow best practices to ensure compatibility with tools and services. Key considerations include specifying the `servers` property in the root object, limiting titles, names, and summaries to 50 characters, avoiding inline schemas definitions, providing non-empty examples, ensuring valid examples conforming to schemas, defining at least one security scheme, using a unique operation ID, returning a 2XX response for GET operations, adding operation level tags, specifying parameters in order, adding descriptions to API components, including contact information, and reusing components to maintain scalability. By adhering to these guidelines, developers can generate better client libraries (SDKs) and developer portals for their APIs using tools like APIMatic's Code Generator and Portal Generator.