If its a Java or object-oriented framework or library - follow SOLID principals and limit dependencies. Make it unit testable. Try to make it consistent - calls to get one type of object should be similar to ways to get other types of data, etc. Functional approach to the interfaces - pass in required arguments, get back result. Don't require user/developer to create several configuration or factory objects before they can call a simple method to get some data, etc. Don't require the use of dependency injection frameworks or other complexities. If its web-based API return data as simple JSON or XML without overly complex namespaces, etc. Name things consistently - the names of response objects and properties should be the same as the corresponding inputs, etc. Copy - look at a successful API or library which does something similar - then copy the way it works and the way its documented. Especially if the users/developers already know how to use that one...
