With a little effort and management, your clay soil can actually be of great benefit to your plants. Clay has a greater ability than other soil types to retain minerals necessary for the production of healthy organic life in your garden. The main difficulties of clay soil are that it provides poor drainage and is difficult to plant, but clay soil can be managed and changed over time.
Break up the top 6 inches of clay soil you presently have in your garden. Swing your mattock and burrow it deeply into the ground, then pull it out. It will bring soil up with it, and repeating this motion throughout your garden will help break it up.
Break up the chunks of clay soil you have dredged up out of your garden. It is easier to mix particles together if they are of similar size. This is an important step to take before you add compost.
Add compost to the soil in the amounts specified by either the manufacturer (if using commercial compost) or your compost recipe (if using homemade compost). Usually, compost amounts will be specified on the basis of coverage in cubic feet.
Mix the compost into the soil. Repeat as necessary after each crop has run its course and you are preparing to plant new ones. Soil conditioning is an ongoing process, and one that is never complete.
Work your soil as little as possible after the initial conditioning. Work more compost in as needed, but do not be concerned with getting it in very deep every single time you do it. If you keep digging deep, you run the risk of destroying the good work you did at the beginning, because the soil will compact itself, leaving no room for plant roots or water.