The modern "Li-ion" batteries are better than the older "NiMH & NiCad" but one thing to remember for ALL batteries is that over time, their charge will depreciate and they will gradually hold less and less charge.
With a new battery of any type, its a good idea to charge it for longer than a standard charge to begin with, usually 6-8 hours will do, even if your phone says the battery is full after only 2 hours, you want to keep it charging for the full 6-8 hours.
Some older rechargeable batteries use to have to be completely depleted before they were charged, however in the more modern batteries, the charging process includes a step whereby the remaining charge of the battery when connected to mains is first discharged, before the battery gets recharged, ensuring longer life.
However, is would still recommend trying to get your battery down fairly low < 10% before charging.
Each charge, will, over time, lead to a build up of residue on the internal cathodes, this will lead to longer charging time, and add to the batteries depreciation thus shortening its life. (as with all rechargeable batteries)