The Kettle Method
This method is a little risky but an be the fastest way to unfreeze a patch of dirt. The idea is to constantly have boiling hot water being poured over the ground in order to heat it up. This is best done using two kettles though one will suffice. The risk with this method is if the ground and general atmosphere is really cold, the water may just freeze and make the ground even harder to dig up.
The Fire Method
This method dates back a very long time when people used to dig graves in Winter using their hands or very rudimentary tools. You basically build and light a fire (charcoal fire would be ideal) over the area of ground you wish to dig and tend to it for a few hours. This warms up the ground and makes your digging a lot easier.
Buy/Hire High Powered Equipment
There is special equipment available on the market for digging holes and for digging holes in Winter. This equipment may set you back but remember you can use it all year for the rest of time (provided they last that long) . If money is an issue you can always hire the equipment out or a company who specialises in the industry. This is the most expensive option but will definitely make your life a lot easier.