For Trusted Research Environments (TREs) to be safe, secure, and productive, they must also be usable. In turn, a TRE that is useable minimises barriers to use and provides a productive and accessible analysis environment for research. Ensuring TREs are usable is a core concern of the Standard Architecture for Trusted Research Environments (SATRE) specification, a reference TRE architecture and accompanying implementation created using a community driven approach. This report contributes to that project in two ways. First, we provide a rich set of recommendations that builders and operators of TREs can follow to increase TRE usability. We encapsulate these recommendations in a TRE usability principle which is incorporated into SATRE's specification architecture version 1.0. Second, we outline the methods and analytic perspectives we have used to understand users' needs and we recommend a series of future research ideas now required to advance this work.