To get my phone dialed for backcountry use (maximize battery life but not limit functions I need in the BC), @ the trailhead I close all apps except Settings and whatever apps I am using: Theodolite, Altimeter+, iTunes…etc. In Settings, I turn off both Wifi & Bluetooth antennas. Under Mail’s settings I turn off Push Notifications and schedule it to Fetch manually. i.e. I turn everything off except “Cellular Data.” Finally, I turn on Airplane Mode.
This setup allows me to—with the flip of a switch (airplane mode)—turn on the GPS antenna and as little else as possible, only when I need it.
To use a Location Services app (e.g. Theodolite), I wake up the phone, switch off Airplane Mode, then use the app. When I’m done, I turn airplane mode back on, possibly close down the app, and put the phone to sleep.
If I’m fastidious about this routine, and I keep it in a zip-lock baggie near my base layer (for waterproofness & warmth), and take it out only when needed, I can have full function of the phone’s camera, GPS, and play tunes all day (6+ hrs, if I’m 100% @ trailhead). If I’m actively tracking myself (e.g. Strava), it will only last ≈ 4 hrs.

In theory, this is simple. In practice, it’s much harder than it sounds. Other distractions arise: a panoramic photo op, fiddling with the baggie, a text message. Texting requires data to remain on and it’s easy to forget to turn it off again. Texts really rankle my system because, “When iPhone hungry for signal, iPhone burn through juice until signal found.” Once you turn Data on, iPhone checks in with The Cloud to see if it missed out on anything. If it learns there’s a text waiting in the hopper, it will use battery, boosting the antenna until it receives the text.

Can anybody beat all day battery? How? Anybody using a battery pack?