API Patterns and Practices?

 
113   96.7
May 28, 2010
 
I am tasked with designing an external API (Application Programming Interface) for our website. Third party users will be able to programmatically login and connect to our backend servers, databases, dataware house data, etc.

Are there any guiding principles, patterns, best practices out there for designing the actual API (protocol, methods, classes, etc)