As S.Lott said, use-cases. They will determine what exactly your API should be directed towards doing. If you design your API to complete a very clear, specific goal - functionally coherent - you are most likely going to end up with an API or component in your API that is both easy to use and understand.
Designing an API should be like designing a user interface. Most all concepts of UI can be embraced by an API, for example, the KISS principle or even Kaizen.
I'd link to those UI concepts, but I'm a new user so they won't let me post more than 1 hyperlink. Good example there: StackOverflow, let us know before we post ;).
