For those that run their machine plugged in for long periods of time (days/weeks), I recommend removing the battery and setting it aside somewhere until you need it. I have noticed with my PowerBook, as well as a few Dell laptops I've owned in the past, this can help retain the capacity level of the battery. I've done experiments using coconutBattery to confirme this.
Over a period of about a month, while leaving the battery in the machine but never actually running the machine off the battery (read: always plugged in) I noticed a decrease in capacity of about 4-5%.
This is of course assuming you never calibrate the battery. Calibrating the battery (usually) restores most, if not all of the battery's original capacity.