If, like me, you had your phone put into a boot loop by the GMail update of early October 2013, here is how I recovered mine. The main problem is that you need to disable sync in the short time between when the phone finishes startup and before it reboots again. On the occasion I tried this the phone was starting from cold and I don't know if it would work when it was between boot loops. I'm not game to re-brick my phone and try. As soon as the home screen displays, swipe down from the top of the screen to display the quick settings buttons (WiFi, Airplane Mode, etc). As soon as it is displayed, swipe horizontally to bring the "Sync" icon into view (the arrangement of these buttons is configurable so you might not need to swipe to see the "Sync" button). The sync button will be lit, indicating that Sync is on. Tap it to turn it off. If you are fast enough, the problem should be gone (you might need one more reboot cycle).