Structured overlay networks for Peer-to-Peer systems (e.g. based on Distributed Hash Tables) use proactive mechanisms to provide efficient indexing functionality for advertised resources. The majority of their occurrences in proposed systems (e.g. Chord, Pastry) provide upper bounds (logarithmic complexity with respect to the size of the graph representing the network) on the communication cost in worst case scenarios and their performance is superior compared to unstructured alternatives. However, in particular (empirically observed) scenarios where the popularity of the advertised resources follows a distribution considerably different from the uniform distribution, structured P2P networks may perform inferiorly compared to well designed unstructured P2P networks that exploit effectively the resource popularity distribution. In order to address this issue, a very simple caching mechanism is suggested in this paper that preserves the theoretical superiority of structured overlay networks regardless of the popularity of the advertised resources. Moreover, the churn effect observed in Peer-to-Peer systems is considered. The proposed mechanism is evaluated using simulation experimesnts.