The healthiest charge for a lithium ion battery seems to be about 50%. If you are going to store your phone for an extended period, charge it to 50% before turning it off and storing it. This is easier on the battery than charging it to 100% or letting it drain to 0% before storage.
The battery, by the way, continues to degrade and discharge if the phone is turned off and not being used at all. This generation of batteries was designed to be used. If you think of it, turn the phone on every several months and top the battery up to 50%.
Tips to extend battery life
The tips above address battery lifespan directly. Battery lifespan is also affected by battery life, how long your phone lasts on a single charge. Improving battery life extends the lifespan of the battery by slowing down those charge cycles.