Charge it straight away out of the box until fully charged, and then run it down until either fully depleted or single figure %'s (better fully deplete for battery's lifecycle). And continue to do so and your battery will last a lot longer than if you always keep it plugged in! (I'm pretty sure you can run direct off mains without a battery for long sessions at home, although would like someone from pcs to verify). Bottom line though, do NOT always leave it plugged in else before you know it you'll be shelling out another £80 for a new battery! .
Edit: What Flucky said too