Remember that when there is no signal most phones use a lot of power searching for one. So turn off at least the cell part of the phone.

Personally I find that using paper maps (possibly augmented by a quick check with the phone's GPS only if needed) is a good way to go. That way you can leave the phone off except when actually needing it to make a call, read/send email or text messages, or to do a quick journal update.

If you must have it on and using the GPS the whole time, then what Cyclebum said pretty much nails it.