The important thing is that you use native plants, especially plants native to your area. They are already accustomed to your soil, your average annual moisture, etc. Furthermore, they will need less (or no) fertilizer. And clay soil is not all bad-it tends to have more nutrients in it than sandy soils. Improving clay soil can be a long-running project, involving getting more organic matter into it, not so much for nutrition, but to help the clay particles separate and allow for more drainage. Providing good drainage, such that water does not collect and stay around roots, as is often the case with clay soils, is essential. We found this article from Fine Gardening magazine on Improving Clay Soils. It was written by Keith Baldwin, who mentions living in the Piedmont. The best we can figure out, Durham is also located in the Piedmont, a plateau which crosses several states, so his experience with clay soils seems to apply to your situation. His solutions are pretty labor intensive, and the article is fairly long, but it would be worth your while to absorb some of his suggestions and modify them to suit your energy and time levels. Another, less intense article, Clay Busters, lists some native plants that will grow in clay soil. We are going to give you a list of plants that will grow naturally in your area, most of which are on the Clay Busters list and all recommended for North Carolina. We've added a couple of native grasses, which are often overlooked but are valuable for adding texture and shape to a perennial garden. When you click on the plant name, it will take you to a webpage describing the optimum conditions for each plant; all that we have selected are perennials.
Finally, here is a list of Native Plant Suppliers in North Carolina. Many more commercial nurseries are now offering at least some native plants, and you can probably find one in your area.