[asguy]

There have been some great responses that cover the gist of it, but to add a couple more mundane specifics:

- Build up the thinnest abstraction that lets you express a solution in the problem domain.

- Allow the API user to express their intent, and not be focused on mechanics they don't need to understand to get the job done.

- Focus on getting the interfaces right before the implementation is perfect.

- A clean interface will allow you to make drastic changes to the implementation without the API consumer becoming aware.