I would highly suggest reading the battery guide that's posted here in the FAQs.
But what you want to do is "keep the juices flowing", in lames terms.
Keeping the MB on the charger won't wear down the battery as you aren't using it, however, keeping the battery topped off for a long period of time isn't good for it either.
Personally, I keep it on the charger as much as possible; however, I know that I take it off the charger every day for a set period of time so the battery is always getting use, even if it's minimal.
If you take it off here and there, don't worry about it, but if you leave it plugged in for months at a time, after calibration you might see some problems.
I think you should go get "coconut battery"; it's a free application that shows the age, health, and charge of the battery.
Very small application and I run it from time to time to see how my battery is holding up.