NEVER let the phone discharge completely - except for the first 3 times - fully charge*, then use it until it shuts off. After that, don't let it fall below 40%.
*Charge it until it indicates 100% - then for an additional 30 minutes. Due to the only way we can measure State of Charge of a lithium battery, the moment it hits 100%, it's really about 98%. And, as Mycrossover said, "overcharging" it is impossible (I've had phones on the charger for days - and the batteries are fine) because the charging circuit in the phone stops when the battery is at a real 100%. So 3 full charges, use it until it shuts off, then never let it drop below 40%. (Start looking for an outlet at 50% and you'll find one before it drops to 40%.)