Overall best practices for testability is difficult, since that could involve whether the code "hides" defects (e.g. race conditions causing intermittent threading issues or difficult corner case issues). Usually testability goes hand in hand with good development practices: well documented code, following a coherent coding style standard, and good communication practices. Once you clear away the "distraction" defects due to inconsistent development practices, it is much easier for the team to address the more severe defects.
If there was any one item that I would identify as an overall best practice, it would be to pay attention to accessibility. You touched on that in #2, but it goes farther than just providing unique screen object identifiers. Developing a well-organized web page structure that allows interaction with external text readers makes the app naturally compatible with GUI-based automation like Selenium and makes overall testing of the app much easier.