Digital signage network (DSN) is capable of delivering customized content to designated screens in a real-time or near real-time manner, which provides tremendous potential for building dynamic demand stimulation tools. However current DSN media buying process is mainly carried out through manually conducted negotiation between the DSN operator and the advertisers. This practice does not capitalize the unique technology advantage offered by the newly emerged advertising medium. We propose automated DSN media buying models which allow advertisers to customize their promotion schedules in a highly responsive manner. Specifically, we design a direct revelation mechanism and an iterative bidding model for DSN promotion scheduling. We show that the direct revelation mechanism computes optimal solutions. We evaluate the revenue performance of the iterative bidding model through a computational study. The implementation of the iterative bidding mechanism is also described.