Sharing knowledge pertaining to software architectures becomes increasingly important. If this knowledge is not explicitly stored or communicated, valuable knowledge dissipates. However, stakeholders will only share knowledge with each other if they are motivated to do so, or in other words if the necessary incentives are created. In this paper we identify three incentives for architectural knowledge sharing: the establishment of social ties, more efficient decision making, and knowledge internalization. Next, we discuss our experiences on how architectural knowledge is shared in a large software development organization. Based on these experiences we propose a set of prerequisites that need to be met to foster successful architectural knowledge sharing. The importance of these prerequisites is motivated by demonstrating that they create the identified incentives.