Good APIs Project

Make your API scalable (i.e. able to cope with a high number of hits), extendable and design for updates. Test your APIs as thoroughly as you would test your user interfaces and where relevant, ensure that it returns valid XML (i.e. no missing or invalid namespaces, or invalid characters).
Embed your API in a community and use them to test it. Use your own API in order to experience how user friendly it is.

“Once you have a simple API, use it. Try it on for size and see what works, and what doesn’t. Add the bits you need, remove the bits you don’t, change the bits that almost work. Keep iterating till you hit the sweet spot.”