You should let your phone's battery drain before recharging.

Apple

A common myth surrounding phone and laptop batteries is that it's always best for the life of the battery to let it drain fully before charging it again.

This is true in some cases. When a device uses a Nickel-Cadmium battery, for example, you'd want to let your phone fully drain before charging it again. Why? Nickel-Cadmium batteries, unlike Lithium-Ion batteries, suffer from what's known as "memory effect." When they are charged and discharged hundreds of times, they start to lose the ability to charge up to 100%, draining your battery life significantly over time.

There was a time when most electronics ran on Nickel-Cadmium batteries. Cordless telephones and answering machines all ran on Nickel-Cadmium. In 2006, most NiCd batteries were replaced with technology that used Lithium-ion batteries. These can be found in all Apple devices and do not suffer from "memory effect" the way NiCd batteries do.

"Lithium-ion polymer batteries have a high power density," Apple says on its website, "and you can recharge a lithium-ion polymer battery whenever convenient, without requiring a full charge or discharge cycle."

Apple does advise, however, that you should let the device go through at least one charge cycle each month to help keep the electrons moving (as opposed to a NiCd battery which needs to go through a full charge cycle every few days). Letting the device drain from 100% to fully shutting off at 0% helps to maintain the life of the battery.