In the context of artistic performances, the complexity and diversity of digital interfaces may impair the spectator experience, in particular hiding the engagement and virtuosity of the performers. Artists and researchers have made attempts at solving this by augmenting performances with additional information provided through visual, haptic or sonic modalities. However, the proposed techniques have not yet been formalized and we believe a clarification of their many aspects is necessary for future research. In this paper, we propose a taxonomy for what we define as Spectator Experience Augmentation Techniques (SEATs). We use it to analyse existing techniques and we demonstrate how it can serve as a basis for the exploration of novel ones.