Thriller, Filler, Spiller is a concept Frankie Flowers, aka Frank Ferragine, endorses when he advises on planting containers. The above diagram, from Portland Nursery's site, demonstrates the concept.
First, start with your thriller. The thriller is the focal point and is usually tall and/or very colourful. Used here is a dracaena but any grasses, millet, etc. will suffice. The more stunning in formation and colour, the better. Remember this is your thriller.
Secondly, select your fillers. These are mid-level fill ins. Something somewhat busy but lower growing, but not sprawling or trailing. Depending on the light and wind, these could be begonias, marigolds, osteospermum, fuchsias, pansies, etc. Go traditional or original, the choices are limitless.
Third, fill in around the edge of the container with your spillers. These are trailing plants that soften the overall appearance of the container, flowing over and beyond the container, anchoring it to the garden or surrounding area. I love to use sweet potato vine, but try bacopa, ivy, trailing osteospermum, trailing geranium, bamboo, licorice vine, etc. Use complementary colours and textures to your advantage to make a stunning arrangement. Here are some more photos for inspiration: