While you can't save hybrid tomato seeds unless you want to put up with the unpredictability of their offspring, you can save non-hybrid or heirloom varieties. Their offspring will remain consistent, resembling the parent plants. You won't need to actually pollinate the tomato plants to create seeds. Tomatoes self-pollinate when the wind shakes their branches in the garden, or when shaken by hand if kept in a greenhouse.
Allow a fruit on a vigorous tomato plant to ripen fully on the vine. Scoop out the seeds, along with the gel surrounding them, and place them in a glass jar.
Allow the seeds to sit in their own juice for four days at a temperature not over 70 degrees Fahrenheit and ferment, making it easier to separate the pulp from the seeds. The fermenting process kills seed-borne tomato disease and mimics the natural rotting of the fruit.
Stir or swirl the mixture daily.
Add water to the fermented mixture to separate the fermented seeds, which will sink, from the pulp, which will float.
Pour off the floating pulp and most of the water.
Pour the seeds onto a screen or paper towel. Leave the seeds at room temperature until they are thoroughly dry.
Store your seeds in a labeled container or envelope in a cool, dry place protected from insects. Label the seeds with the name, variety and date of collection.
Place the seed packets in a jar. Place a small cloth bag filled with ½ cup of dry powdered milk beneath the seed packets to absorb moisture. You can also use a silica gel desiccant.
Seal tightly and place it in a refrigerator. They should keep for four or five years.