The most accepted way to hard reset your Windows Phone is to go to Settings>About and tap Reset Your Phone.
If you are a bit contrarian however Winuser.ir has discovered another way which has a bit more flair.
In the dialer simply type in:
You will not need to press any other button, and the above warning will pop up. As can be seen from the description, the reset will not delete your maps data or presumably network settings, which means it is likely ideal for doing where you do not have a lot of connectivity and are away from home, and can not download 100’s MB of maps.
The feature apparently works on most Windows Phones, but will obviously not function if your device is unresponsive, and also does not work from the emergency call function.
Thanks Winuser.ir for the tip and research.