There are a few considerations to keep in mind when creating a place for butterflies, whether it's adding a one-gallon container to the front porch or revamping the entire backyard.
First, make it eye catching. Butterflies are attracted to large bright flowers or showy clusters of smaller blooms. Mixtures of pink, yellow, and white work best. Mass them together to form a striking collection that will catch the eyes of passing insects. Avoid reds, though, since most insects can't see the color (which explains why hummingbirds tend to pollinate red flowers).
The reason butterflies visit flowers, of course, is to drink nectar, their fuel source. Their long tongues, which can be as long as their bodies when uncoiled, help butterflies reach deep into tubular flowers to tap nectar sources unavailable to most other insects. Thus the plants that attract butterflies usually have nectar rewards in deep tubes.
Butterflies also need big landing platforms, since most species of butterflies prefer to land while they drink nectar (a few are proficient at hovering during meals). Most members of the sunflower family are suited to butterflies; the showy outer petals surround the nectar-producing flowers in the center. Flower clusters work the same way -- the butterfly can stand on one or more flowers while probing adjacent tubes.
Besides flowers, there are other "props" that will attract butterflies. Many species will sip at the juices coming from rotting fruit. You can put a small amount of wine in a dish with a bit of smashed banana to achieve this effect. In addition, male butterflies are particularly attracted to mud puddles (they are looking for salts that precipitate out of minerals in the soil). A sunken wet saucer with horticultural sand works well to attract salt-seeking males. And finally, many species of butterflies like to perch on dark flat surfaces, where they can bask and watch for potential mates. A flat rock in a sunny spot will provide for this need.
One good way to become familiar with the common butterflies of your neighborhood and the plants that attract them is to take a walk on a sunny afternoon and look for gardens that are attracting butterflies. You may discover that your neighbors are already practicing the art of butterfly gardening, and most gardeners are happy to share helpful advice -- and cuttings, too.