Keep it at 60-80% and in cool temperature. Only fully charge it when you need to go away for maximum battery life. Avoid fully discharging the battery. Repeat full charge and full drain kills the battery faster.
Many laptops nowadays only charges the battery to 50% - 80% capacity unless you override to charge to 100%.
The basic theory is that the more often the battery is charged to 100%, the sooner the battery will die. This has to do with the chemistry inside the battery itself. You can say that the chemicals become less effective to retain a charge the more often you use it.
Not being able to replace the battery yourself kinda sux, but that's how Ultrabooks (and tablets) are designed.