How to break in a new cell phone battery?

You may need to recalibrate the device for the new battery, especially if you got a 3rd-party replacement:

1. Allow the phone to drain completely to the point where the device no longer powers on
2. Plug the device in and allow it to charge completely. Do not use your device during this time.
3. After the device charges completely, keep the device connected to the charger for an additional 2+ hours.
4. Repeat this process once more to ensure that the device is being charged to maximum capacity.

If this doesn't cure your battery problems, make sure you don't have apps running in the background wasting power. Culprits include zealous Twitter clients and apps that constantly use the GPS.

Is it a lithium polymer battery battery? Discharging it probably won't have any effect:

This used to be true for NiMH batteries, where discharging fully was thought to help extend both voltage depression effects (where cell voltage would start to drop more quickly when in use) and general capacity (the "memory effect").

Fully discharging cells cures voltage depression in NiMH cells, but if you fully discharge a whole NiMH battery pack then the stronger cells in the battery will "reverse" the weaker ones. The weaker ones go flat first, and then get charged backwards by the others. This is bad for the weaker cells, and will kill a NiMH battery pack more quickly. Model enthusiasts will use a proper electronic discharger that discharges the cells without damaging them.

There is no need at all to purposefully discharge a lithium polymer battery. The chemistry is totally different. Each cell in a lipo pack has a working voltage between 4.2v (fully charged) and 3v (fully discharged). If you run any lipo battery based RC models then you'll know how carefull you have to be.. fly your lipo helicopter for a few minutes after the power has dropped each time and then youll have to chuck the battery away within a few charges as youve completely messed it up! A laptop will switch off when any of the cells in the pack reach 3v to protect the battery. Then when it is charging it will charge each cell individually and balance them all out. There is no memory effect though.

Model enthusiasts preserve your lipo batteries when not in use by keeping them in the fridge. Even better if they are discharged half way (apparently).

Totally impractical for a laptop battery, but a cold laptop might be nice on a warm day.

(Lifted from this discussion on laptop batteries, but pertinent anyway)

You don't need to "break-in" a typical cell phone (lithium-ion) battery. It doesn't help the battery at all.

Having said that, as Kyle mentioned, calibrating it may be necessary.