If you leave you macbook connected to the charger, eventually you will see that the computer knows to 'disengage' from the charger and use up some battery (the batery charge level will show 100% a few hours later the level will be at 99% and falling.) That being said, you should disconnect the macbook from power and run it totally on battery power at least once a month. Everyone has a different opinion on how low to run the battery before recharging. I use my macbook on the go almost everyday, and the battery runs down to between 30 & 20% on a regular basis. My machine is 3 years old and the battery is in perfect health, about 94% of a new battery.
It will not harm your battery at all to leave it connected at all times, for the most part. That's how I use mine, plugged in 24/7 unless I need to use it on battery. When I do need to use it on battery, I can easily get 6+ hours on a 2+ year old battery. (Of course, note that that's just doing basic text file editing and a bit of web surfing with the screen brightness turned down.)
As already pointed out, you do want to use it on battery now and then for best performance. But, beyond that, I don't worry too much about it. Use your computer as the tool that it is, and use it the way you need to use it. If the way you need to use it means you have to replace the battery sooner, better that than not getting the most out of the machine. You don't want it to be like the car you love but never drive, because you're afraid to take it out of the garage.
Everyone has an opinion but Apple has recommendations: http://www.apple.com/batteries/notebooks.html
It is impossible to overcharge a MacBook battery.
All batteries eventually wear out and require replacement. It will require replacement sooner if you do not use it.
It really is best to not obsess about this. Just use the computer as it is designed - to be used, not pampered.
Leaving it plugged in does not cause any problems with charging. In fact, it is required that lithium-ion chargers stop before reaching even the slightest amount of overcharge, because overcharge is actually a serious safety hazard. It is simply not allowed.
Just don't let it empty fully on a frequent basis. That is bad for it. It's much better if it's topped off after you drain it part way, instead of letting it continue to empty.
The only risk to the battery when plugged in is if the machine is running hot for a long period of time (because it's working hard in a game or appilcation). High heat will shorten the life of a fully charged lithium battery. I'll usually unplug and let the charge drop a bit if it's hot.
But like the others say, it's not worth your time to micromanage the battery too much. Use it the way you need to use it.