If you're growing non-hybrid beans in your garden this year, it's very easy to save a few beans to plant in your garden next year. Beans very, very rarely cross-pollinate, so even if you haven't gone through the process of isolating your beans to maintain purity, you can still save the seeds and be confident that you'll end up with the same type of bean you grew this year. It's very simple:
1. Be sure you're growing a non-hybrid seed.
Hybrids are labeled as such on the seed packet. A hybrid bean won't come true from seed, so if you want to be sure you'll get exactly what you planted this year, make sure you're saving seed from non-hybrid plants.
2. Let the beans dry completely on the vine. This is best done near the end of the growing season, because once you start letting the pods mature, the bean stalks slow down quite a bit, and you won't get much of a harvest.
3. When the pod is completely dry (it will be light brown in color, generally) remove it from the plant and open it up, revealing the dried beans inside.
4. Remove any chaff or pieces of pod, and store your dried beans in a labeled jar, envelope, or other container in a cool, dry place, and plant the beans next year.