This Battery University Article has a summary that states the following
•A portable device should be turned off while charging. This allows the battery to reach the threshold voltage unhindered and reflects the correct saturation current responsible to terminate the charge. A parasitic load confuses the charger.
•Charge at a moderate temperature. Do not charge below freezing.
•Lithium-ion does not need to be fully charged; a partial charge is better.
•Chargers use different methods for “ready” indication. The light signal may not always indicate a full charge.
•Discontinue using charger and/or battery if the battery gets excessively warm.
•Before prolonged storage, apply some charge to bring the pack to about half charge.
•Over-discharged batteries can be “boosted” to life again. Discard pack if the voltage does not rise to a normal level within a minute while on boost.
I would also add that heat is the single worst enemy of any battery. Keeping it cooler is always better. Not cold, just cooler. Any usage will cause heat, and don't worry about that, leaving it plugged in can generate a lot of heat at times.