A full discussion and overview of RESTful API design could easily take up a full chapter. The following is a summary of the key RESTful design goals and how these apply to REST APIs delivered via Apex (some guidelines are handled for you by the platform). As we discuss this topic further, the use of the term operation represents the methods in the application's Service layer classes. This will be used to help expose the application's REST API but should not steer its design.
There is no one list of guidelines, and some are open to interpretation. However, if you enter Designing a RESTful API in Google, you're sure to come up with some useful reading. I recommend that you focus on those articles that give insights into how some of the larger services, such as Facebook, LinkedIn, and Salesforce themselves, interpret RESTful, such as http://www.apigee.com/.