iOS: Your GPS and 3G connection drain your battery faster than anything else, so if you can avoid using either for navigation purposes you can avoid a dead iPhone. Crowsflight sets a plan for the places you want to go, downloads the relevant maps, and only uses your compass to help you reach your destination.
While nowhere near as robust as any turn-by-turn navigation system, if you need a low-power method to get where you want to go you'll find that in Crowsflight. You just enter a location, watch it pop up on your list, and set out on your way when ready.
The phone will tell you which direction to go. While you'll undoubtedly prefer to navigate with proper directions when in a car, if hiking or on foot you can use Crowsflight to find your way without killing your iPhone's battery.