Boot Mac OS X Into Single User Mode by holding down Command + S on system boot.
Once the system has booted into its terminal looking single user mode simply type the following:
NOTE: Where it says USERNAME below simply replace that with your Mac OS X main admin login username.
$ mount -uw /
$ rm -R /Library/Preferences/$ rm -R /Users/USERNAME/
$ /bin/launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist &$ dscl . -delete /Users/USERNAME$ rm /var/db/.AppleSetupDone
How-To: Easily Enter DFU Mode [Device Firmware Upgrade Mode] For Jailbreaking or Restoring your iPhones Firmware.
Simply plug your iPhone up to the computer that you sync it with then hit play on the video below and do exactly as it shows the very second you hit play.
If you follow the instructions above and your iPhone screen is black then your iPhone IS successfully in DFU mode and iTunes should launch and tell you just that, Otherwise reboot your iPhone and follow the video above exactly as it shows until iTunes tells you that you are in DFU Mode.
Congratulations Your all done! Your iPhone is now ready to be load any version of iPhone firmware you like (Unlocked, Jailbroken, Current standard firmware or past releases)