Airplane mode and a battery pack works great. I think a tip is if you upgrade your phone get the best you can afford they have better battery efficiency. I can go all day recording tracks and waypoints with mine but I paid a lot when I upgraded.It really is time for me to get my head out of "how I used to do it" and get into the 21st century. I guess my concern was battery life but I can get a decent battery pack to plug into my smartphone when it gets low. I was planning on carrying spare batteries for my GPS anyway so it won't be an issue to carry a pack for the phone.
Thanks for the posts I appreciate the input, and yes, looking at Onx website it does appear to have GMU's as a feature.
Get a battery pack thats at least twice the MaH rating as your phones battery. The packs MaH rating divided by your phones battery MaH rating will give you an estimate on how many recharges you will get in the field. I usually subtract 1 charge just to be safe...