Understanding the mechanisms of increasing the number of physics teachers educated per year at institutions with thriving physics teacher preparation programs may inspire and support other institutions in building thriving programs of their own. The Physics Teacher Education Coalition (PhysTEC), led by the American Physical Society (APS) and the American Association of Physics Teachers (AAPT), has supported transformation of physics teacher preparation programs at a number of institutions around the country for over a decade. In 2012-2013, PhysTEC supported an independent study on the sustainability of its sites after project funding ends. The study sought to measure the extent to which programs have been sustained and to identify what features should be prioritized for building sustainable physics teacher preparation programs. Most of the studied sites have sustained increases in the number of physics teachers educated per year as well as funding for physics teacher preparation. About half of the programs are thriving, in that in the post-award period, they have further increased both the number of physics teachers educated per year and funding for physics teacher preparation. All studied sites that sustained increases in the number of physics teachers educated per year have two features in common: a champion of physics teacher education and institutional commitment. The thriving physics teacher preparation programs in this study implemented different elements of physics teacher preparation according to diverse local priorities and opportunities, including the unique expertise of local personnel.