The thriller-filler-spiller design concept is my favorite for containers. The design makes such a big impact and takes a lot of the guesswork out of planning a container planting.
You start with tall thriller plants that add a vertical element to the combination. If your container will be viewed from one side, plant the thriller in the back. If it will be visible from all sides, stick it in the middle. Some of my favorite thrillers are switchgrass, angelonia and coleus.
Use more rounded plants as fillers to give the container the look of abundance, such as lantana, spurge and SunPatiens. If the thriller is in the back, place the filler flowers in front of and around the thriller in a U-shape. Otherwise, place them all around the thriller.
Finally, spillers go in. These trailing plants soften the edge of the container and balance the height of the thriller. I like to use sweet alyssum and petunias. Depending on how your container will be viewed, plant a couple of spillers in the front or use several spillers all around the edge of the container.