I use the "Topo Maps" app on the iphone. It uses topo maps that are downloaded to the phone beforehand and therefor does not need cell service or wifi.
For absolute maximum battery time, don't run GPS apps with map display. I use the "Tomtrails" app that only dumps coordinates. I like the convenience of the UTM coord dump and use that with a real map that also has a UTM grid. I tend to print out custom hi-res maps of where I am going with UTM grid overlays and laminate the paper in plastic to weatherproof them. I can then just write on the plastic with a Sharpie - use diff colors even
