There may be easier ways to do it, and we don't exactly have clay soil, but we found a good old fashioned pickaxe to do just fine in preparing our rocky, hard, dense sandy soil to be moved or removed by shovel.

Get you a five or six foot crowbar. You know...the kind with a point at one end and a round knob at the other end. You can get it a Home Depot or Lowes. It's heavy and it does a great job of breaking up the dirt so you can get it out with the post hole diggers or shovel. It also keeps you in good shape. Good luck...DT

Gas powered auger/post hole digger.

in our rock hard clay i have foudn the easiest way is to run a soaker hose in the area for a couple days before you plan on digging by hand. just turn it on low and let it run. then the day before turn it off. next day the soil is dry enough not to be a mud hole, but damp enough to dig easily. it may take a few times to get the flow right, too much water and you have a muddy mess and not enough and you wasted the water. the trick is to do it slow enough that it does not run off.

toolwise, i use a pick axe, a shovel and an old post hole digger. my PHD is not the squeeze type, it is an auger that you turn to dig the hole. it has a T-handle on it and you just spin it down. when we put up a fence at work several holes had to be hand dug due to proximity to underground utilities. i could dig 2 holes to 1 against the guys using the squeeze type PDH.