Humus rich materials, as much as you can afford or get hold of, as often as possible, twice a year at least to start with, autumn and spring. Humus rich materials means composted animal manures (unless you want to grow root crops like parsnips), soil conditioning composts, garden compost you've made yourself, leaf mould, spent mushroom compost, anything like that, but not potting compost. If the clay is really thick and claggy, the addition of grit (horticultural grit) would also be useful. Most clay soils, though, aren't necessarily nutrient poor, particularly in comparison with light sandy soils, it's just they're so difficult to dig. The solution for both types of soil is the same - humus. If you can turn it over in the next few weeks, before November, adding humus material, then leave it in clumps and hope for a frosty chilly winter - frost breaks down the clods for you.