Are you one of those people who make the battery run low before charging your device? You're not alone -- there are still some folks who think that you have to do that to prevent the memory effect. There is no memory effect on lithium-ion batteries, so stop that! I personally recommend charging every night when you go to sleep.

On my previous phone, if the state of charge is at... say... 20%, the phone would actually turn off a second or two after ringing. That is, the screen, rumble pak, and speaker placed a big enough load on the battery to make the voltage sag down to unsafe levels (and cause the phone to turn off).