There are different Do's and Don'ts that depend on the battery chemistry. Li-ion batteries tend to lose their overall lifespan when stored charged. They don't have any issues with getting partial charges, aside from your Time Remaining indicator getting out of whack. Li-ion batteries are also capable of exploding if overcharged, thus the charging circuitry for Li-ion batteries monitors the power level of the battery so as not to overcharge.
It does not matter if the battery is left in the notebook while on AC power. Not only does the battery act as a sort of UPS while installed, but the notebook will NOT charge it if it's full. Unless you want to circumvent that design and force it to overcharge at your own risk.
Li-ion batteries are rugged compared to Ni-Cad. They do have a finite lifespan, though. I would recomend using Notebook Hardware Control to force your power settings to be quite miserly while on battery power. Dell's QuickSet can disable your wired network connection to save a bit of power, too. Disable Wi-Fi if you won't be using it. And as for the largest power drain of them all: Dim the screen.