I'm assuming that you mean how to maximize how long you can use the device (or have it in stand-by). If so, in addition to what Zebulon says, turn the screen brightness down as much as you can tolerate.
If you meant that you want the battery to be usable for as many months/years as possible, then you should generally charge it up to about 85% charge or less and recharge it when it gets to around 30%. To keep the battery meter working, you'll need to charge it to 100% and drain it to <5% every so often (some people say once a month).