Jeanie, you WANT to run it down atleast once a month. This ensures all of the cells are being drained and recharged. Constantly charging it as much as possible, leaves only the last few cells being used and the others dying.
This goes for phones, iPods, what not
I've had two batteries for my D600, the origina 53w/hr one which went from about 3.5hr > 60 or 90 min run time. My 62w IIRC is starting to get pretty low again. Then again, i blew a ram module in dec as well..