When I travel I have been using the New Trent Battery to keep my iPhone and other USB powered devices charged up. It has two USB ports so I can charge my iPhone and my Bluetooth headset at the same time. The battery is about the size of the iPhone 4, and turns on with the press of a button. If nothing is connected or the charge cycle is done the battery will turn off automatically.
I found this product works quite well and I don’t have to replace my case or make my iPhone overly thick.
I got my New Trent IMP70D from The AT Guys for $49.99.