I was really inspired by pictures of gardens designed by Gertrude Jekyll, and she had either written, or was quoted saying that she designed to appear that the plants occurred there naturally, in meandering clumps and drifts. Although she designed gardens for formal estates, often in precise perennial borders, the plantings within the borders had that natural flow to them, one clump appearing to meander into the next.
I do tend to plant in odd numbers, and do find that plantings are more striking when plants are in clumps or drifts rather than in lines. Lines seem to emphasize the stems and foliage, whereas clumps mass the blooms together for a bigger visual impact of that color we're going for.
Gertrude Jekyll was also very masterful at color harmony. She said, I believe in her writings, that she concentrated cool colors toward the center of a planting, and warm tones toward the ends. I'm not sure how exactly that method worked so well, but it certainly did make for amazing gardens.
When I have something blooming that is playing well with it's neighbors, I carry a bloom around the garden, and hold it next to various other plants. Often I find gorgeous combos I would never have considered otherwise. Sometimes a contrasting eyezone color will be beautifully highlighted and emphasized by another plant, which is one of the reasons why I think Pamela Harper's color echoing strategy works so well.
