The expanding need for an open information sharing infrastructure to promote scholarly communication led to the pioneering establishment of arXiv.org, now maintained by the Cornell University Library. To be sustainable, the repository requires careful, long term planning for services, management and funding. The library is developing a sustainability model for arXiv.org, based on voluntary contributions and the ongoing participation and support of 200 libraries and research laboratories around the world. The sustainability initiative is based on a membership model and builds on arXiv's technical, service, financial and policy infrastructure. Five principles for sustainability drive development, starting with deep integration into the scholarly community. Also key are a clearly defined mandate and governance structure, a stable yet innovative technology platform, systematic creation of content policies and strong business planning strategies. Repositories like arXiv must consider usability and life cycle alongside values and trends in scholarly communication. To endure, they must also support and enhance their service by securing and managing resources and demonstrating responsible stewardship.