You need to exercise your battery. Every month or so, use it off the charger and let it die -- kill the battery. Otherwise leaving it plugged in 24/7 will actually decrease the battery life over time.
This is NOT TRUE!
Read up on how batterys work now days.
What you are saying is accually the worst thing you can do for your battery.
You really shouldn't blurt out these things.
My MBP is four years old with the original battery, so I'm not at all surprised that its capacity is down.
But are you sure you're actually supposed to let Mac batteries die completely on purpose? I've let it happen by accident, and that seemed to lower the capacity pretty quickly. I lost 5% in a month.
The Lenovo I'm borrowing is all sorts of wonky. It doesn't always recognize the extended battery and shows a lot less time left than there should be.
Yeah 4 years takes is toll. There are some ways you can make sure you get the most out of your battery tho.
Here are some good tips if you want to read about how to take care of you battery the best way.
http://marketingsucesso.hubpages.com/hub/How-to-increase-laptop-battery-life
Im still surprised that Apple haven't added a function to set max recharge of the battery to 50% or 80%.
Since many keep their laptops plugged in all the time. This area between those numbers are the best place to keep the battery loaded, to get the most out of its life.
That is why many laptop brands have a function to lock charging to not go over one of those amounts.