Working with a conservation group to preserve and restore wild areas is one way to help maintain butterfly habitats. Creating butterfly gardens is another way-and it's fun for school groups and individals.
Before you start your garden, do a little research to determine what species of butterflies are common in your area and which host plants they prefer. In general, your garden should include a variety of plants that bloom at different seasons, be in a sunny area, and be protected from the wind. Remember to include plants for larvae as well as adults.
Expect the larvae to EAT your plant leaves, so don't spray them with insecticides.
For a list of some common butterflies, their host plants, and the habitats where they may be found in San Diego County see http://www.sdnhm.org/research/entomology/sdbutterflies.html.