Distributed hash tables (DHTs) provide a means to build a completely decentralized, large-scale persistent storage service from the individual storage capacities contributed by each node of the peer-to-peer overlay. However, persistence can only be achieved if nodes are highly available, that is, if they stay most of the time connected to the overlay. In this paper we present an incentives-based mechanism to increase the availability of DHT nodes, thereby providing better data persistence for DHT users. High availability increases a node's reputation, which translates into access to more DHT resources and a better quality-of-service. The mechanism required for tracking a node's reputation is completely decentralized, and is based on certificates reporting a node's availability which are generated and signed by the node's neighbors. An audit mechanism deters collusive neighbors from generating fake certificates to take advantage of the system.