Your battery is most likely a litium-ion battery. The best way to charge lithium-ion cell phone batteries is when they are 2/3 to 3/4 depleted. It is the older nickel type batteries that should be fully depleted before recharging. Like one of the other posters said...it is best to put on charger each night before you go to bed.
You should charge your phone when the battery is almost completely depleted. If you charge it frequently when it still almost a full charge it will create a short memory and then need charging more often.
Every night before you go to sleep put it to charge
Charge it when ever it is flat lol