Yes leaving it plugged in all the time is not a good idea.
Believe it or not the best way to keep your battery life to its longest is to use it till its almost empty then charge up fully then remove the charger.
Its the nature of these type batterys called 'memory state' here is some info on the subject.
http://en.wikipedia.org/wiki/Memory_effect
Although it discusses NiCad's it applies to nearly any rechargable battery.
The batteries are made up of a series of cells hooked up end to end in .. well series. If one of the cells should fail it usually fails to a dead short becomming in effect a piece of wire, furthermore it drops the overall voltage increasing the currrent compounding the problem.
If it were possible you 'could' tear apart the battery case and replace the failed cell. However with the new Lithium or poly batteries they are manufactured so that they cannot be seperated. Once a cell fails the only option is to replace the entire pack. Be careful to get an exact match as these days these battery sets are alot more than just batteries. They have current limiting and overcurrent protection built in.
Good advice, charge your batteries to full capacity, then unplug them. 'Excercise' the battery by discharging fully and recharging fully. Just dont leave it plugged in for days on end.
Even more important dont 'store' the battery in a discharged state.
In the meantime Ive dealt with these folks in the past with decent results
http://www.laptopbatteries.com/
Might also want to try Ebay.