I have a Pocket Power Bank, which is a battery recharger for 4AA Ni-MH batteries. When there are charged batteries in it, you can plug a USB device directly in to it and it will start recharging the attached device. If theres no outlets within a long distance, you can stock up on some regular AA batteries, stick them in the charger and use those to power your phone/mp3 player/personal massager/etc. Cheap and light, and it means you can leave all your heavy proprietary power adapters at home and just bring the power bank along with whatever specific coupling wire you need for your device.
http://www.semsons.com/pocpowban.html