If you have clay soil, you already know that it can be difficult to work with. However, other kinds of soil can be difficult in different ways, and clay soil holds some advantages over other types. Its water and mineral retention capabilities are unmatched, which means that once you have fixed it, it is not difficult to have healthy soil for your plants. Luckily, the process of fixing it is not terribly difficult.
Test your soil to find out what kind of clay soil you've got. Some clay soil can benefit from calcium (which lime and gypsum can provide), while other soil can benefit from phosphorus (which is easily provided by bone meal). If your soil is too alkaline, the test kit will indicate the need to apply sulphur.
Break up the soil as much as possible with a sturdy digging tool such as a mattock, hoe, or spade. Use whichever you prefer, but make sure to dig down to a depth of at least six inches when you are turning the earth over.
Mix compost and whatever amendments your soil test recommended into the soil you have just turned over. Make sure to read the directions for your compost and your agricultural amendment materials, because they will specify what amounts should be spread over what area of land. If you are using compost you have made yourself, make sure to read the recipe you have used, which should specify this information. Mix this all together thoroughly.
Rest your amended soil for at least a week prior to planting anything in it. Not only will the microbial flora and fauna have a chance to properly adjust if you give them this time, but you will spare your new plantings the agony of having their roots burned by too high a concentration of the nutrients you have just used to amend their home.
Plant ground cover mulch (also called "living mulch") plants around your main plants in your garden. Choose attractive ground cover that will complement whatever it is that you are planting. This will not only help to deter weeds, but when the season is done, you can allow the ground cover to wither and turn it over into the earth, providing yet more vital nutrients to your clay soil.
Repeat this process every growing season and amend as necessary. You should notice that it continues to improve.