Conference presentations are stressful communication tasks for many speakers. This mental stress inhibits the speaker's ability to recall information and perceive the audience. Moreover, stress deterio- rates linguistic and paralinguistic capabilities of the speaker. This paper proposes a wearable talk assistant to monitor mental stress and pro- vide relaxation feedback during public speaking. The assistant senses the speaker's body stress by means of heart activity. With this data the system recognises stressful talk phases. We evaluate the approach in authentic conference talks. The talk assistant was worn by 5 speakers be- fore, during, and after giving a 20 minute talk. Our results demonstrate that it is feasible to distinguish the talk period from the surrounding periods and detect talk phases. These findings show that heart activity provides vital information to estimate the speaker's body stress. More- over, we outline ways to proactively support a speaker non-disruptively while talking in order to maximise the presentation performance.