Mix plenty of organic matter into the soil you dig out of the planting hole. Throw about a cm of the amended soil back into the hole and scratch it in to make a transition layer: any sudden change in soil texture can stop the drainage. Be sure to leave about 1/2 cm of the original root ball showing once it is planted: Clay soil easily smothers the bases of the stems when it's piled up against it. Top dress with organic matter twice a year to maintain good soil structure, and lots of earthworms.

If you go to the bottom of the page you'll see the alphabet strung out from left to right - click on C and select clay plants, or clay/sun plants, for ideas.