First, keep in mind that every plant's seeds are not worth saving. Hybrid plants, those created by cross breeding separate parent plants for specific features, often produce sterile seed or seed with such a high degree of genetic variability that the resulting progeny may not resemble your original plant at all, such as seen with the strawberry plant (Fragaria x ananassa). Because of these genetic possibilities seed from hybridized plants are not recommended for saving.
Second, many garden plants are open pollinated. That means that their flowers do not self pollinate but instead are pollinated via insects or wind, or a combination thereof, by nearby plants of the same family. Open pollinated plants include many cucurbits (squash, cucumbers, melons), brassicas (cabbage, chard, broccoli, mustard greens, spinach cauliflower, kale, radish) and a variety of other plants such as beets, onions and basil. These open pollinated plants will naturally cross with others plants of the same family growing in their vicinity, so in order for them to reproduce identical plants from seed they must be isolated from all other plants in their family that may pollinate them. It is very difficult, if not impossible, for home gardeners to isolate plant varieties and avoid unwanted cross-pollination as these plants must be secluded by large distances to maintain the original variety. Having plants with more variety from year to year may not sound like that big of a deal, but be aware that this cross-pollination will likely result in the gradual deterioration or "running out" of the seed to be harvested.
A third concern is that of seed transmitted diseases. If a disease infects a plant at the end of the season it may not cause much visible damage at the time. But, if seed is saved from the diseased plant, it may result in severe injury to or even death of young plants in the next season.
If after heeding these warnings you are still interested in trying your hand at seed saving, here are a few pointers on how to do so successfully. To begin, always harvest from the best plants of each type. Select disease-free individuals with the qualities of traits, such as flavor, color, size, shape, harvest time or flower you find desirable. Remember that in order for seed to be viable, it must be mature. This will likely mean that the fruit you take the seed from will be beyond the stage where it is desirable to eat. So, you will have to decide which fruits to harvest to eat and which to let fully ripen to harvest for seed. Seed set tends to discourage further fruit production as well as reduce the overall vigor of a plant, so you'll want to wait until the end of the season for each particular plant type to harvest seed.
Typically seeds are ripe when they have turned from white to somewhere between cream colored and dark brown, depending on the plant. Flower seeds are mature when the flowers have faded and dried. This will apply to many vegetable plants that you may not think of as flowers, such as lettuce, carrots, onions and most herbs. Plants with pods, such as beans and peas, have mature seed when the pods have turned brown and dried. Most seeds of fruiting plants, such as tomatoes and peppers, will be ready once they are past the peak of ripeness. The catch to this is that if you wait for all of the seeds of a plant to ripen you may lose most if the seeds to birds or animals, so you'll want to try to harvest when most of the seeds are ripe.
Once seeds are harvested, they must be properly prepared in order to be viable. Most seeds of pods and flowers should be prepared by the dry method. This means that the seeds should be allowed to mature and dry on the plant as long as possible. After harvest, complete the drying process by spreading them in a single layer on a screen in a well-ventilated area with low humidity. You will know they are ready when the chaff (the dry, scaly protective casings of the seed) can be easily removed by blowing or gently rolling the seed in your hand. For very small and lightweight seed, such as lettuce or carrot, you may want to put the seed in a paper bag to dry as the paper will help wick moisture away from the seed while also containing the seeds.
Seeds harvested from fleshy fruits should be prepared by the wet method. This is done by scooping out the seed masses and soaking them in a little bit of warm water in some sort of container for three to four days, stirring daily. This will allow the seed coatings to ferment, a process which kills some diseases and separates the good seeds, which will sink to the bottom, from the bad ones and the remaining fruit pulp, which will float at the top. You can then pour off the pulp, water, bad seeds and any other undesirable material that may have floated up. Remove the good seeds and spread them on a screen or paper towel and allow to dry completely, otherwise they may mold in storage.
Now your seeds are ready to be stored. You can put them in envelopes, jars or plastic freezer safe containers, making sure to label containers with the seed type, variety and date harvested. Toss them in the freezer for a day or two to kill off any pests that may have hitched a ride. Finally, store the containers in a cool, dry location. Keep in mind that seed viability will decrease with time. Most seed should be used within three years of harvest but others, such as corn, onion and parsley, must be used within one year.
I wouldn't recommend planning your entire garden around saved seeds, as many things can go wrong, but it can be a great way to supplement catalog or store bought seeds and save yourself a few bucks in the process. Start small and see how it works for you. And remember, seed saving isn't only good for preserving unusual and heirloom varieties of vegetables, but also for many flowers and native plants as well!
