Heat is a major cause of battery degradation, partially why I don't agree with constantly charging/discharging since it causes more heat.
Keep your laptop cool, perhaps even consider battery saving options while plugged in if you find it is generating too much heat.
Edit: A running laptop will cause heat, so shut it off when not using it. A plugged in laptop shouldn't create much more heat than a laptop running on battery power, if anything, the battery should be cooler when plugged in and fully charged (assumption made).
Keep your laptop cool, perhaps even consider battery saving options while plugged in if you find it is generating too much heat.
Edit: A running laptop will cause heat, so shut it off when not using it. A plugged in laptop shouldn't create much more heat than a laptop running on battery power, if anything, the battery should be cooler when plugged in and fully charged (assumption made).