My desire is to used the RWGPS iPhone app as a navigation device for long rides (brevets) rather than having a separate GPS. Obviously battery life is a big limiting factor. I like to have the screen on so I can verify I am on-route, even though that results in a shorter battery life. On a recent 300K I noticed that my external battery drained much more quickly than I expected after testing it on a previous ride. Later I realized that I had enabled the "Auto-Rotate Map" setting.
My tip is to leave this setting off to extend battery life. I like having the map oriented to my direction of travel, but I am guessing that this feature uses the magnetic compass. Smartphone compass hardware uses a surprising amount of power.