Browse around and take mental note of what is available. Browse through some of the websites listed in the references section and pay attention to their designs. Many gardening books will layout the container with blue prints, showing you which plants go where. You can form your own, unique container based on what you see and what you know about the plants you like. See local guides or nurseries for plants that grow well in your area.
Containers often have a theme.
Themes can include colors, plant styles, contrasting colors, etc. Below are some plant suggestions for working with colors. It isn't necessary to have a container entirely of flower plants or one entirely of shrubs. Mix and match different types of plants such as annuals and perennials. Try to plant winter, autumn, spring and summer varieties all together in one large container.