Xeramtheum: I think the bread knife could work in some limited cases, but here in the Southeast archeologists use a plain round nose Razorback shovel and cut the tip straight across at about 10 cm. This slight dip that gives you the power to scoop out shovelful after shovelful. With the sharpened razor sharp edge you can shovel-skim a flat surface or dig 4 x 4 m square holes hour after hour day after day--building A Schwarzenegger legs. The razor sharp edge makes it easy to punch out a straight edge. Any bumps are smoothed out with a sharpened flat mason's trowel. These two tools, along with a bastard file to sharpen tool edges, will save your back, and allow you to cut a box-shaped excavation with precision edges even in clay soil.
in clay soil, we wet it down the night before and cover with black plastic. The next day its ready to dig.