This is a question I've been asked quite a bit lately, and it seems to be a common problem in many gardens. Put simply - clay soils have a very strong bond with water, and tend to get waterlogged, with very little space for oxygen for the roots to breath. The result being, that rot can set in very quickly, or at the least, plants tend to look unhappy and become diseased easily.
The simplest solution would be to 'soften' the soil by adding coarse sand (as much as is possible). The shape and size of coarse sand leaves lots of gaps for air, and makes the soil less 'sticky'.
Another way to fix clay soils, would be to add large amounts of compost or well-decomposed organic matter - this does the same thing as adding sand, but it will also improve the soil's fertility at the same time. The only problem is that you would need to add quite large quantities to see the equal 'softening' effect.
Also, bear in mind that clay soil compacts very easily when wet, and doesn't bounce back up the way sandy soil would. Because of this, make sure that when the soil is wet that you try to walk or run wheelbarrows over it as little as possible.