In the past, I used to carry spare smartphone batteries. I would carry an extended battery or a standard spare in my bag. However, I stopped that trend last year and now I carry a small external battery pack that allows me to recharge my smartphone when I'm traveling.
Given that I replace my smartphone roughly every 2 years (through the Verizon "New Every Two" plan), I found that it really didn't make sense for me to go out and buy many accessories for my smartphone. Therefore, I no longer buy spare batteries. I also don't buy cases. I simply use a screen protector and that's it.
By using an external battery pack, I know that I can also use that same power source to recharge my iPod, my Bluetooth headset, and a few other small electronics. Plus, when I replace my smartphone, I can still use the same battery pack. I recommend getting something small and something that simply has a standard USB port. This little 5V outlet can recharge many devices as long as you carry a USB cable.