Pick colors that complement your porch. Contrasting colors clash and show lack of outdoor design appreciation. Try to find flowers in the same general color palette as the rest of your house. If you have blue cushions on your furniture, use lighter and darker shades of blue flowers. Also, take into account the color of your house and any other home accessories, including shutters.
Look for flowers and plants that are native to your area. Flowers that have trouble growing in your climate are a challenge because they tend to die off quickly or need replacing often. Native flowers continue to bloom and some even bloom for multiple seasons before you need to find replacements.
Use planters of different sizes and shapes. Find planters that work with the design scheme of your house, such as simple terra cotta pots for wicker furniture and a country look. Planters also come in marble, granite, stone, copper and aluminum, as well as different colors. Using different sizes and shapes give guests multiple things to look at when they arrive.
Place the plants along the outside edge of the steps, leaving a small gap on the side. You want enough space on one side that it doesn't interfere with your walkway or path and enough space on the other that you won't accidentally nudge the planter and cause it to tip over.
Prune the flowers frequently. There's no point in decorating the porch steps if the flowers die off and you leave them alone. Continue feeding and watering the flowers regularly and replacing any dead ones.