With the development of generalized connectivity and the increased use of Body Area Networks, wearable antennas have become ubiquitous in the last decade. Their design presents a challenge, as they per definition are located at the interface between free space and an environment which is intrinsically lossy and hostile to the efficient radiation of electromagnetic waves. In this contribution, we propose a theoretical analysis of different canonical sources placed on biological tissues. Based on the insight gained, several simple design rules will be proposed and illustrated on a practical antenna design.