Li-Ion degrades with use, so you should prefer shallow discharges. I.e., always charge it as soon as possible. This is in contrast to Ni-Cd and Ni-Mh accumulators, which do like to be fully discharged from time to time to keep their full performance.
So normally you should plug in your laptop whenever possible.
Another thing that is bad for Li-Ion is heat, which unfortunately laptops produce. So ideally, you'd remove the accumulator if you don't need it, and store it half-charged in a cool place. But many laptops don't work well/at all w/o their accumulator, so that's more theory.
The battery prefers a partial rather than a full discharge. Frequent full discharges should be avoided when possible. Instead, charge the battery more often or use a larger battery. There is no concern of memory when applying unscheduled charges.
Lithium-ion batteries should not be frequently fully discharged and recharged ("deep-cycled").
You may need to discharge it fully occasionally to recalibrate the capacitiy measuring electronics in the accumulator. Every 30 cycles or so should be enough.