http://www.adafruit.com/products/1566 is sold specifically for this purpose, and it has some interesting notes about how it behaves (like output fluctuation when the battery is simultaneously charging and providing power) that might be worth noting. If all you do is run off a battery, then unhook the battery to charge it, you shouldn't have those sorts of issues.
It has to reliably provide 5 volts and at least 700mA (depending on your Pi model and peripherals). The Pi is more sensible to the voltage than the amperage.
See http://elinux.org/RPi_VerifiedPeripherals#Power_adapters for reliable information about that.
The Raspberry Pi forums are filled with "90% of the time it's the power supply not up to the specs, please check the specs" messages for any power and `random crashes´ related problems.