Nectar if the key when it comes to trying to attract butterflies to your yard.  The more nectar in a flower, the more popular the plant will be with butterflies.  But, just as humans have their favorite food, each type of butterfly has it's favorite as well.
So, your first step should be to determine which species are native to your area and what types of flowers they are attracted to.  Try looking in the National Wildlife Federation info site at
(www.nwf.org/backyardwildlifehabitat/pdfs/butterflies.pdf <http://www.nwf.org/backyardwildlifehabitat/pdfs/butterflies.pdf>) or the National Resources Conservation Service site at (www.nrcs.usda.gov/feature/backyard/butterfl.html <http://www.nrcs.usda.gov/feature/backyard/butterfl.html>) if you would like to do more research.

Regardless of the plant, there are some basic rules of thumb that will make any garden more butterfly-friendly:

        Flowers should be located in a sunny location - preferably with five to six hours of sunlight each day.
        Plant in an area that typically doesn't get a lot of wind - that way the butterflies don't have to constantly battle the breeze while they try to feed.
        Place a few stones in the garden in a sunny location - butterflies like these spots to take a break and warm their wings.
        Have a birdbath or a shallow saucer of water near your flowers to provide the butterflies with the water they need.
        Avoid using insecticides in your garden - butterflies are inscets, so while the insect sprays can kil other pests, they will also likely kill the butterflies as well.
        Plant for blooming throughout spring, summer and fall to provide a continuous source of nectar.
        Be patient - while butterflies can identify their favorite plants from miles away, it may take awhile for butterflies to find the "harvest" you've planted in your yard.  While you're waiting for them to arrive you can still enjoy the beautiful blossoms of the flowers you've planted.

Keep in mind, you can't have butterflies without caterpillars, so you need to "feed" them  as well.  They have specific food requirements, so when you learn what species are common in your area, tailor your planting accourdinglyu.  Learn how to identify the larva of butterflies so you don't dismiss them as other pests and remove them from your garden before they have a chance to transform.

While each butterfly has its favorite flowers, many butterflies are attracted to plants that boast red, yellow, orange or purple flowers.  In addition, flowers with flat-topped or clustered blossoms with short flower tubes allow the butterflies to reach the nectar more easily.
 Here are some good choices to consider:
        Achillea
        Aster
        Goldenrod
        Lavender
        Lilies
        Purple Coneflower
        Thistle
        Violets
        Zinnia