One of the great secrets of outstanding gardens is underplanting. The idea is to plant layers of plants like nature does in a forest. For example, using smaller plants such as ferns, hosta, or perennial geraniums under the canopy of a tree or group of large shrubs.
My picture shows our big walnut tree behind the house. The space under it is a steep slope that used to be covered with weedy lawn grass that was dangerous to mow. (I almost overturned my big Toro mower there a couple of times.)
To start the garden on the slope, we killed off the grass one summer, and then began planting shrubs and perennials that are walnut-tolerant - mostly viburnum shrubs, daylilies, hosta divisions from the shade garden, Solomon's seal (the plain green version and the variegated one), and sweet woodruff, also moved from the shade garden. For early spring, there are tulips, daffodils and grape hyacinths. The area is coming along nicely, but I still have some gaps to fill.
For more planting inspiration, Margaret Roach (garden writer and former editor of Martha Stewart Living) has a great post with lovely pictures all about underplanting. Her blog, which has only been around since March, has become one of my almost daily must-reads. As a gardener, she's a true artist.