I'm ordering an external battery pack.
Trent Battery Pack
Seems to me to be the best of both worlds. Phone stays slim, I can use conventional cases, and have more juice than an extended battery. I can also offer a bit of charge to others in a pinch. Just connect it once or twice during the day when not using the phone. They come in different capacities, some have two USB ports and can be used for both my phone and tablet.