The below is taken from the web. If you Google 'improving clay soil' you'll find many more links.
''Improving your clay soil will take a bit of work, but the good news is that the work you do will instantly improve the structure of your soil and make it easier to work with. Most of the work is done up front, with some annual chores to continue improving your soil.
It is best to improve an entire planting area all at once. I often see advice about just improving individual planting holes as you go along, but I don't recommend this practice. When you dig a planting hole in clay soil, then plop in a plant and nicely amend only the soil you use to backfill, your plant will be happy for a little while. But what you've essentially done is make the planting-hole equivalent of a flower pot. Eventually, the plant will start sending out roots, but when they reach the limits of the nicely amended soil you backfilled with, they will have a hard time expanding into the hard clay around them, and will start circling around in the planting hole instead. You'll end up with a perfectly root bound plant, and it won't grow as large or as healthy as it should.
Decide how much area you want for your new garden. If you are improving an existing bed, you'll just have to dig out any plants you want to keep, and then you can get started. If you are preparing a brand new bed, there are a few more steps to consider.
To improve your soil, you'll need to add six to eight inches of organic matter to the entire bed. You can add any organic matter you can get your hands on. Grass clippings (as long as they haven't been treated with chemicals), shredded leaves, rotted manure, and compost are all perfect choices. Spread your organic matter on top of the soil. Here's where the manual labor comes in. The organic matter needs to be mixed into the top six to twelve inches of soil. Digging it in and mixing it with a shovel is a great way to do this, as it moves a lot of earth without pulverizing the soil particles the way tilling can. However, if digging is just too hard on your back, using a tiller is a fine method.
When you're finished, your garden bed will be several inches higher than it was originally. It will settle some over the course of a season, but the soil structure will keep improving as microorganisms in the soil work to break down all of the organic matter you've added. The bed can be planted immediately, however. You'll be adding more organic matter on the top of the bed once or twice a year. This will continue the process of improving the soil's structure and offset any settling that happens.''