Double Digging 

On a new site with deep topsoil, dig a trench 2 feet wide to a depth of two spades, and add some organic matter to it. Move the soil to the end of the plot. Dig a second trench of the same size next to the first-use the soil to fill the first trench. Add organic matter to the second trench, and repeat these steps across the whole plot. If you have thin soil, dig to just one spade's depth and fork the bottom to a further spade's depth to avoid bringing infertile subsoil up to the surface.