The emphasis on network-centric operations for the next generation air traffic system presages an important opportunity for collaborative work. However, there is no clear design guidance for collaborative work across such a large and complex network of users and operators. In this paper we will describe the Multi-center Traffic Management Advisor, a system which can enable collaboration on regional air traffic problems. We will describe specific findings regarding collaborative work from a four-year testing period with six separate air traffic facilities, including a design feature we believe was crucial for improving collaborative work. This work has important implications regarding the future collaborative environment of the U.S. air traffic system as well as for collaborative work across large, complex organizations.