Good mixed container design has at least one thriller, filler, and spiller. Thrillers are the big, bold plant in the center or back. They can have a vibrant color pattern, large leaves, or a tall spiky grass. Fillers are the smaller plants that surround it. They have a contrasting texture and/or color to the thriller. The spillers are the vines that spill over the edge of the container for a cascading effect.
A mix of colors, texture, and height is always good. Flowers are nice but the foliage should be able to hold it's own when the plants are done flowering. Go to a local garden center (not a big box store like HD) and mix and match plants in your basket. See what looks good next to each other as a dry run. You can fill in with cheap fillers from big box stores if you need to fill a big container. To keep your plants healthy don't fill your containers completely. Let the plants fill in themselves over the growing season. The more soil each plant has the better they will look come fall.
My front porch is pretty shady so I have begonias and fuchsia cascading over the sides. Actually one of the (non hardy) fuchsias overwintered and is already huge. Not bad for a 99 cent plant.
Foxi's advice about thriller, filler and spiller is great. I'd try out different combos at the nursery to play up different aspects (ie gold variegation with gold flowers) until you find one you like. If you have a cooler climate, I'd plant annuals a little closer together than recommended because it'll take longer for it to fill in.