Planning an annuals garden in advance is the only way to make sure that an annuals bed is color balanced and that the plants work well together in terms of sun or shade, height, and soil.
If you list your favorite plants on paper first, noting their available colors and cultural requirements, you're off to a good start. As you narrow down those that work well together, you can actually see a workable garden emerge in front of you. By taking this extra bit of time, you can save yourself from being disappointed later.