All the old "memory effect" charging myths are related to old Ni-Cad batteries (nickel cadmium)
Modern Lithium Ion chemistry does not suffer from memory effects and doesn't need to be charged down to low states in order to get a good full charge later on.
Rules for Li-Ion
-Avoid letting your battery state go very low or dead for long periods of time... this is the most harmful state for Lithium Ion chemistry batteries. YOU ARE BETTER OFF IN THE LONG RUN IF YOU CAN CHARGE YOUR PHONE AS OFTEN AS POSSIBLE... DONT LET IT GET BELOW 30% IF POSSIBLE AND YOU WILL HAVE A LONGER LASTING BATTERY IN YOUR PHONE... SEE CHART:
DOD=Depth of Discharge
100% DoD 300 – 500 cycles
50% DoD 1,200 – 1,500 cycles
25% DoD 2,000 – 2,500 cycles
10% DoD 3,750 – 4,700 cycles
what this means is that if you fully deplete your battery (100% depth of discharge) you will only get 300-500 cycles,
if you only deplete your battery by half, you will get 1,500 cycles.
-Charge to 100%, but don't keep the phone at 100% either... Don't let it sit on the charger for much longer than it takes to get to 100%... (although this probably isn't necessary with the logic that apple puts into their charging products... It stops charging the battery automatically.)
-If turning the phone off or putting it in storage... keep in a cool place at about 40-70 percent charged..
-Never leave your phone in a hot car... worst thing for a battery
Source:
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries