Adding another dimension of variety to your container
Leaves come in all shapes and sizes, from big and bold to almost needle-like leaves. Some plants are so finely textured they are almost see-through. Many, such as astilbe, are fern-like and complex. Grasses can be fine-textured and spiky at once.
A good rule of thumb is to choose one plant with bold leaves, and mix it with one with complex leaves - the two textures show each other off to perfection (and we're going for perfection here). Adding a spiky-leafed plant and one with a medium rounded or heart-shaped simple leaf form will give you a container that is interesting even without the flowers.
But flowers, too, can be big and bold, like zinnias, or tiny and fine like candytuft. Some flowers are fluffy, some stark. The mix makes each more exciting. Make sure you also have a good mix of flower-textures - let a few bold ones act as star players with some smaller, finer flowers as supporting players.