Creating a high quality WordPress Theme is about more than just a nice design. What’s going on under the hood is equally important: using the right Template files, writing clean code and testing for all kinds of content.
Luckily there is a wealth of information, plugins and other helpful tools available on the web. This is an overview of the best resources to help you develop Themes according to the best practices.
Fränk Klein composed a handy list of resources on WordPress Theme Development Best Practices. Definitely something to bookmark.