Nowadays, the popular adoption of smart phones has enabled the development of several location-based applications such as indoor localization for shopping centers, airports, hospitals, among others. Typically, these services are based on Wireless Local Area Networks (WLANs), where the selection of access points (APs) plays an important role for the localization accuracy. In this paper, we evaluate different criteria of AP selection as well as the number of required APs, the impact of AP power transmission and the usage of different frequency bands. The results are obtained by means of experimental evaluations that provide more realistic comparisons. Finally, we provide novel guidelines for the design of improved indoor positioning systems.