The first step in saving bean seeds is to make sure you are getting them from a well-established heirloom variety so you'll have the best chances at success. There are lots of heirloom bean varieties available to choose from. For pole beans, the Kentucky Wonder Pole is probably the most common. For bush beans, the Blue Lake 274 variety is one of the more commonly grown. Early Bush Italian is a common heirloom variety of flat bean, while Purple Queen is a common heirloom purple bean.
For best results, you want to leave the beans on the plant until they dry out. The pods will turn brown and brittle and when you shake the pod, you should be able to hear the hardened seeds rattling around inside. At this stage, you can go ahead and pick the beans and bring them inside to dry out a bit more. Be careful when picking the pods as they sometimes use this opportunity to burst open and spill the beans onto the ground. Also, when harvesting, make sure to place the beans inside a bag or other container that doesn't have any holes as inevitably, there will be lots of bean seeds rolling around on the bottom. Once you've picked the dried beans, you can then remove the bean seeds from the pods. We like to bring the beans inside the house and take them out of the pods there. We take the busted up pods and toss them in our compost pile.
At this point, you can spread the seeds out on foil or a baking sheet and let
them air dry for another couple of days. You want the seeds to be hard
and shriveled up a bit. Then you can then put the seeds inside an
envelope and store them in a jar in your refrigerator. Make sure you
label the envelope so you know what seeds it contains. If you are
saving lots of different seeds, several envelopes can fit inside one,
quart-sized jar. It's also a good idea to put a tablespoon of dry rice
at the bottom of the jar just to absorb any moisture.
Click on the following links to learn more about growing green beans.