Usability engineers & researchers are crucial parts of User Experience. While not as "glamorous" as designers they bring evidence to the world of design. Through observation of heuristic tests they offer empirical evidence that a design is working, failing, what users like, what users aren't finding, what users are doing that they don't even realize they are doing it, etc. Usability is the closest thing design has to being a science.
The attached technical paper goes through a variety of topics with usability in mind and makes recommendations. It links out to studies and research already done supporting best practices.
The basic findings and recommendations in this document are that: