Thrillers, Fillers & Spillers
"Thriller, Filler, Spiller" is a common scheme for planning show-stopping patio pots by incorporating plants with complementary growth habits. Put one large "thriller" in the center, surrounding it with one or more "fillers" and a spiller which will grow downward, anchoring your container.
Thrillers are your "stars"... usually tall, upright plants with especially dramatic flowers, colorful foliage or a unique shape that lasts all season long. Examples of some good thrillers: Canna, Spikes, and ornamental grasses. For a smaller, more subtle container, Coleus or Angelonia may be thrilling enough!
Fillers tend to be mounded plants which add texture, color, and mass while hiding the base of the thriller. If your container is large enough you don't have to choose just one! You can opt to go with leafy fillers (Coleus, Dusty Miller, Persian Shield, Rex Begonias, Plectranthus) or fillers bursting with flower power (Angelonia, Cuphea, Lantana, Pentas).
Spillers tumble out of your container, softening its appearance. Spillers can be be leafy (Alternanthera, Creeping Jenny, Ivies, Sweet Potato Vines) or very floral (Bacopa, Nasturtium, trailing Petunias and Calibrachoa). Spillers often look great in hanging baskets!
Next-Level Container Strategy
Variety of texture and form: The fillers and spillers can echo the thriller to get a monochromatic effect, or stretch into adjacent or complementary colors on the color wheel. You can go with several plants of these colors which have different flower shapes, forms or sizes. Adding some interesting texture (for instance, frilly coral sedum) or something in a neutral white (like a small-flowered Euphorbia) or near black (ornamental sweet potato, Coleus) doesn't hurt, either.
Matching size and requirements: Things will eventually go wrong if all of the plants in your container don't enjoy the same amount of sun and water (specifically, the amount of sun and water your container actually gets!). While you're still planning, make sure all of your plants will still fit in the pot you've given them in a couple of months -- Thrillers love to get big! Also, containers tend to look tidier if everything is proportional... fillers 1/3-2/3 the height of the thriller is a good rule of thumb.