Claude Moore Farm in Sterling, VA had complied some info exactly on this topic. Here's what I found.
To create a really rocking butterfly garden you need both host plants (for the larvae to house eggs and caterpillars to eat after hatching) and nectar sources (for the butterflies to drink). Both host and nectar plants are essential. Each species of butterfly likes different plants. Here are 10 plants that are assured to lure those pretty creatures in:
1. Butterfly Bush (Buddleia)
2. Milkweed or Butterfly Weed (Asclepias)
(host and nectar for Monarchs)
3. Purple Cornflower (Echinacea Purpurea)
4. Zinnia - single flower preferred
5. Mexican Sunflower (Tithonia Rotundifolia) or Coreopsis
6. New England Aster (Aster - novae angliae)
7. Parsley, Green Fennel, Rue or Dill
8. Joe-Pye Weed (Eupatorium purpureum)
9. Red and White Clover (this is also a great grass alternative)
10. Dogbane (Apolynum)
And for even more info, here's a great book on Attracting Butterflies and Hummingbirds to Your Backyard.
Give the little butterflies some shelter from wind by planting trees, or offering a Butterfly House. Add some large rocks for them to sun bath and warm up. Create an area for them to drink water by filling a jar cap with water, sand, and some twigs for them to perch on.
Then you can join a butterfly society and make a contribution to science by sharing your butterfly observations.