In this paper we present a method for improving the precision of an RSSI-based energy-constrained localization system employed in an IEEE 802.15.4 sensor network. The goal application is localization of people in dynamic indoor environments. We introduce an approach which divides the anchor nodes into groups and assigns a path loss exponent to each group. The results from the conveyed tests in our building show a location error smaller than 3m, despite the low energy constraints. Moreover, we provide a hardware platform independent system suitable for both standard and proprietary solutions