[MENTION=210772]Looigi[/MENTION]'s answer is pretty complete, but I'd like to add to it anyway. NiMH and NiCad batteries were around for a long time, and it became well known that they had a memory effect. It was helpful to discharge them fully as often as possible. LiIon are the opposite. Discharge them fully as little as you can. If you have to run one down every so often, don't worry about it, but don't do it on purpose, at least not frequently.

It seems to me that age is the biggest factor in how long a LiIon battery lasts, whether it is used lightly, heavily, or not at all. They last three years.