Spicy Gadget Roll has a piece with some good tips on how to extend the life of your Li-ion batteries:
Battery Calibrating – There is some benefits to fully discharging your lithium battery periodically, for laptops especially this can be important. If you start to notice your battery meter is becoming more and more inaccurate, it may be time for some battery calibration. By allowing your lithium battery to fully drain, this will help the battery recalibrate allowing for more accurate measurements of battery life. This should be done once every 30 charges or when you notice battery readings are off.
But as they point out, DON’T listen to people who tell you the battery will last longer if you fully drain the battery before the first charging. This is related to nickel-based batts and their “memory effect,” which doesn’t occur with lithium.