When choosing plants, consider the ultimate height of the planting compared to the height of the container. Visually, a pleasing proportion is one-third container to two-thirds plant height. In other words, the plant material may be twice as tall as the visible part of the container. Conversely, when featuring the container, the proportion is reversed - the container will be twice the height of the plants. This "rule of thirds" is commonly discussed in the creative arts. Take into account how rapidly the plants will grow, as the proportion will change over time.
Using odd numbers of plants usually works well. With odd numbers, things on either side balance something in the middle. Artistic gardeners often choose odd numbers of plants: (3, 5, 7, 9) to create this symmetrical balance. Sometimes four flowering plants look good with a fifth foliage plant. Experiment with several flowering plants to one interesting foliage plant.