Although a rare occurrence, it may happen, while attempting to jailbreak your phone, that you end up stuck in a infinite reboot loop after restoring your custom firmware to your phone. This obviously poses a bit of a problem because you will end up unable to enter DFU mode (also known as recovery mode) on your iPhone via the usual process:

1. Hold Power + Home Button for 10 seconds
2. Release Power button, and keep holding the Home button for another 10 seconds

The above might not work, and you will simply turn off your phone after step 1.

if this happens what you need to do is quite straightforward:

1. Turn your iPhone off. You might need to hold the Home + Power buttons for about 10 seconds to achieve this.
2. Make sure your iPhone is unplugged from the USB port in your computer.
3. Now that your phone is off, hold the Home button.
4. Reconnect your phone to USB port. It should now turn on. Do NOT release the Home button.
5. Be patient and hold the Home button while the phone boots up, it should eventually enter DFU mode.

The above has saved me a couple of times. Don't despair, I hope this also works for you.