Techniques for controlling the transmission power of wireless mobile devices have been widely studied in ad-hoc and cellular networks. However, as mobile applications for wireless sensor networks (WSNs) emerge, the unique characteristics of these networks, such as severe resource constraints, suggest that transmission power control should be revisited from a WSN perspective. In this work, we take an experimental approach to examine the effectiveness of power control for WSN applications that involve mobility at human walking speeds. Furthermore, we propose two light-weight transmission power control schemes to improve energy efficiency and spatial reuse. The first is an active probing based scheme that adjusts transmission power based on (the lack of) packet losses and applies to all low-power radios. On the other hand, the second scheme requires radios that offer link quality indicators (LQI) to estimate the proximity between the transmitter and receiver. We evaluate both schemes using mobile nodes in an indoor and an outdoor environment. Our results show that the energy efficiency of the proposed transmission power control schemes can be very close to that of the optimal offline strategy. Moreover, our schemes significantly reduce the interference to unintended receivers and improve spatial reuse. To our knowledge, this is the first work that evaluates the effect of transmission power control in mobile WSNs.