There are many things you can do to improve your clay soil. If you
live near a sugar beet processing factory you could get a load of sandy
soil delivered and spread it on top of the clay. This comes from the
washing of the beet which is grown in sandy soil. You will probably
find it has a slight molasses smell to it. Apart from that I would
suggest you need to encourage worms to live in your soil and that you
get as much compost or manure material into as you can lay your hands
on. Don't go to the bother of digging in the compost though, just
leave it on top of your clay soil and the few worms that you do have in
the soil will start to feed on it and drag it down into the soil very
soon making a good quality top soil. If you can get straw bales at a
reasonable price try spreading straw on top of the clay and letting it
rot down. With a thick enough layer it will also act as a mulch to
depress weed growth. Once again the worms will drag the decaying straw
down into the earth and improve the quality of your soil.