Ansible provides a number of best practices on their website at http://bit.ly/1ZqdcLH.
One emphasis in their documentation is on using roles:
"One thing you will definitely want to do though is using the "roles" organization feature, which is documented as part of the main playbooks page. ...You absolutely should be using roles. Roles are great. Use roles. Roles! Did we say that enough? Roles are great."
Creating roles is a key component in making Ansible modular enough so that you can reuse your code across services and playbooks. To demonstrate a proper structure, we are going to create a role that our playbook will then call.