There is a long tradition for crafting wearable objects directly on the body, such as garments, casts, and orthotics. However, these high-skill, analog practices have yet to be augmented by digital fabrication techniques. In this paper, we explore the use of hybrid fabrication workflows for on-body printing. We outline design considerations for creating on-body fabrication systems, and identify several human, machine, and material challenges unique to this endeavor. Based on our explorations, we present ExoSkin, a hybrid fabrication system for designing and printing digital artifacts directly on the body. ExoSkin utilizes a custom built fabrication machine designed specifically for on-body printing. We demonstrate the potential of on-body fabrication with a set of sample workflows, and share feedback from initial observation sessions.