+1 on the cable, but this works too.
Press and hold power and volume down until the screen blinks. Release the buttons and quickly press all three buttons (power, vol up and vol down).
When the bootloader menu pops up, plug in the charger.
Use the volume down button to scroll to the SBF option and use volume up to select it.
Press and hold the power button for about 3 seconds and release it. This will put your phone in the "powered down/charging" mode.
Wait until the battery charges all the way then try again.
I had to try this three times before it worked, so don't give up on the first failure.