Home gardeners were perpetuating and improving vegetable varieties through seed selection long before there were commercial seed producers. Garden plants are wind, insect or self-pollinated. Seed saved from self-pollinated crops are most likely to come true to variety.
Over many decades, people created heirloom varieties by selecting and saving seeds from their best plants, season after season. You can define "best" to be whatever characteristic you're after: biggest or healthiest plant, biggest, tastiest, most interesting or colorful tomato. Whatever you want, just choose the best examples of what you're saving. For instance, if you have two plants and one is sickly, the other healthy, choose a tomato from the healthy one. The sickly one may be carrying some weakness in its genes, so you don't want to propagate that. This is not a time for "waste not, want not." You want the best. Similarly, choose tomatoes in their prime, not over-ripe or under-ripe, diseased or misshapened. To maintain good genetic diversity, it's best to save seeds from multiple tomatoes, and preferably from more than one plant of the same variety, if possible.
Select good quality frruits
FermentationYou don't absolutely have to ferment the seeds, but it makes the seeds easier to separate from the gel, helps sort out bad seeds and reduces some seed-borne illness. If you're going to trade seeds with other people, it's considered good etiquette to ferment your seeds. Cut the tomato in half and scoop the seeds and gel into a small container labeled with the variety name. The seeds are in gelatin like pockets
Seeds are in gelatin-like pockets
Set the rest of the tomato aside for eating. Add a half-cup of water to the container and set it aside in the garage or shed, for 3 to 5 days. The odor can get quite strong. A moldy film will form on top; that's okay. To separate the seeds, carefully remove the film and mold. Then add some more water and stir. Viable seeds will sink, so carefully pour off the water and the floating bits of pulp. Repeat until all the pulp is gone and you have clear water and clean seeds.
Drain them well; I pour them through a coffee filter. Spread the seeds in a single layer on the filter to dry - usually 24 hours is sufficient for them to dry thoroughly. If you're saving multiple varieties, be sure to label the filters.
Coffee filters work well for draining seeds
Once the seeds are thoroughly dry, place them in an enevolpe for storage. Tomato seeds remain viable for years, even stored at room temperature. But for extra protection, you can store them in the refrigerator or freezer.
Coin Enevolpes work well for storing seedsSeed saving is essential for maintaining unusual or heritage vegetables. It is a great way to propagate many native plants too. There are numerous seed saver exchanges, clubs, and listings in magazines like Organic Gardening. Although you shouldn't base your entire garden on saved seed, you may want to give seed saving a try.