I carry a USB battery in my laptop bag; it's good for two full charges of my iPhone. It loses about 10% of its charge over three months.

My BOB contains a AA-powered iPhone charger along with a bunch of Energizer Lithium AAs. If I have to make an emergency call, I should have power. If on the other hand I'm faced with a long-term emergency that keeps me from using AC, 12-volt or USB power sources, I'm going to have to keep my phone turned off most of the time and use it sparingly.