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Mobile terminals with built-in GPS receivers are becoming more and more available, thus the public deployment of location-based services (LBS) becomes feasible. Upcoming LBS are no longer only reactive but getting more and more proactive, enabling the users to subscribe for certain events and get notified when e.g. a friend approaches or a point of interest comes within proximity. However, power consumption for continuous tracking is still a mayor issue with mobile terminals. In this paper we define this problem and propose solutions for an energy- efficient combination of GPS and GSM Cell-ID positioning for mobile terminals. We introduce several strategies for extending the lifetime of the battery and show how these strategies can be integrated into existing middleware solutions. Simulations based on a realistic proactive multi-user context confirm the approach.