There are three things that are important to preserving plant diversity. First, there must be protected areas to house populations of wild cousins to modern plants. This allows returning to find disease-resistant varieties and breeding stock. Second, seeds from a wide variety of heirloom plants should be saved. The seeds must be grown out on a regular basis to maintain germination and viability.
Storage does no good if the plants are never grown.
Third, farmers should grow rare and heirloom varieties on their land. This is where you can be of the most help. Seed saving also allows you to develop a unique strain that is particular to your farm and management practices. If you haven't done it before, seed saving is easy. Order a heirloom variety from a seed catalog, or talk to neighbors or farmers at your Farmer's Market about obtaining seed.