Clay soil is notorious for having drainage problems, which -- over time -- can have serious consequences for plants you plan to grow. To improve the drainage in your clay soil, improve the soil's overall texture and density by amending it with organic matter. Adding the organic matter at the end of your growing season gives it enough time to release important nutrients into your clay soil before the planting season arrives. Make sure you mix the organic matter with your clay soil as deeply as possible to get the most benefit.
Collect organic matter for your clay soil. Look for an assortment of high-nitrogen and high-carbon ingredients in your kitchen and around your yard. Popular nitrogen materials include cow or horse manure, coffee grounds, tea leaves, vegetable peels, fresh grass clippings and old fruit; carbon-rich materials include old hay, straw, dried grass clippings, dead leaves and sawdust. Aim for about half of your organic matter to be rich in nitrogen; the rest of it can be carbon-rich materials.
Shred and mix the organic matter together. Tear or chop any large pieces into smaller chunks no larger than about 1 to 2 inches in diameter. Use a manure fork to mix the carbon-rich and nitrogen-rich materials together in a large pile next to your plot of clay soil.
Prepare the clay soil for your organic matter. If you've already used the clay soil for gardening, remove any weeds or old vegetation, leaving the soil bare. If the clay soil hasn't been prepared for gardening and is still covered by sod, remove the sod completely with a shovel, a labor-intensive job that will work best if you have others helping you with it.
Spread the organic matter across the clay soil with a garden rake. Create a solid sheet of organic matter about 6 to 8 inches thick across the entire surface of your plot of clay soil.
Mix the organic matter thoroughly with the clay soil, using a spade or rototiller. According to the Oregon State University Cooperative Extension, a spade is typically your best option because it allows you to mix the organic matter as deep as 12 inches into your soil, which will greatly improve your soil's drainage. Although it requires less manual labor, rototilling typically tends to damage the soil structure.