Decoupling capacitors -- usually around 100 nF -- are usually placed as close as possible between the power and ground pins of an IC to absorb spikes on a power rail from interfering with the operation of the IC.
Your Raspberry Pi should already have these capacitors added to all noise-sensitive components, so you don't need to add any.
The battery voltage is the next concern. The Raspberry Pi is expecting to be supplied from a USB charger providing 5v @ 700 mA or more. While this is fed into three linear regulators to supply internal 3.3v, 2.5v, and 1.8v rails which are used by virtually all IC's on the board, the 5v rail is also brought out to several I/O connectors.
So you should not input a voltage higher than 5v to the power jack of the Raspberry Pi if you have devices connected to either General Purpose I/O headers P1 and P5, the HDMI connector, or any of the USB connectors. And even if you are not using any of these devices, inputting a much larger voltage than 5v (e.g. 12v) will cause the three linear regulators (3.3v, 2.5v and 1.8v) to get hot since they will have to drop a lot more voltage.
So depending on what voltage your battery is (I assume it is not 5v), you need to provide an external regulator (either linear or preferably a switcher) to bring it down to 5v. At the output of that regulator, you can add a couple hundred μF capacitor.