Clay soils can prove to be the most fertile of soils. It benefits from the ability to retain nutrients and water. It is rich in ingredients needed for plant growth and will benefit from a variety of techniques. Firstly it should be dug in winter with the clods left exposed. The crumb structure can be improved by applying lime or use gypsum if you don't want to raise the alkalinity of soil. Don't work the soil when wet or sticky and try not to compact the soil by walking on it. Realistically it takes several seasons to create a better medium from a clay soil, however to choose plants that thrive in or at least tolerate heavy clay is a sensible starting point. Always delay planting until spring to reduce the damaging effects of cold, wet soil which cause fragile new roots to rot. Dig and mulch with lots of organic matter such as garden compost, leafmold and well rotted manure.