If you are leaving the 3G on and Sprints signal is weak in your area it is going to cause the phone to constantly search for a better signal and drain the battery much faster than normal.
Not sure if this is your cause, but I discovered that 'Google Now' killed my battery extremely quickly. You could try disabling it for a day or so and see if it helps. Of course, I can't remember if that's a feature of the current OTA or if it came with 4.2, since I flashed a custom rom immediately after activating my phone.