Take these steps to prepare clay soil for planting:
• Dig the area in the fall. Leave the surface rough to allow frost to penetrate during the winter months. The freeze–thaw action of the weather does much of the cultivation.
• Apply lime to the newly dug soil in the winter. This changes the charge on the clay particles, making them stick together in groups, forming larger particles. This results in soil with more open, crumbly texture.
• However, lime raises the coil pH, so is not good for alkaline-intolerant plants.
• Add coarse horticultural grit and plenty of organic matter to further improve heavy clay soil.
From the Horticulture’s Gardener’s Guide to Shrubs by Andrew McIndoe.