Saving tomato seeds from your garden is a thrifty way to prepare for your next crop. The best seeds to save are those from heirloom tomatoes; seeds from hybrid or cross-pollinated fruits usually grow, but with unpredictable results. Select the healthiest tomato on the healthiest plant to harvest for seed. Remember that whatever grows from the seed will be very much like the parent plant.
Select a tomato that is ripe, almost to the point of being over ripe. Cut it open horizontally across the middle with a knife, and squeeze the pulp, juice and seeds from each half into a small glass or plastic jar.
Add a little water until all the seeds are covered. Stir this mixture with your fingers.
Cover the jar with a piece of damp cheesecloth, and secure it with a rubber band. Place the jar in a warm location. Stir the contents and re-moisten the cheesecloth every day.
Wait four or five days until you can see that the gel casings around the seeds have dissolved; the mash in the jar will turn dark and form a scummy coating on top. Scrape off the coating with a spoon each day, taking care not to disturb the seeds. You may have to add a little water to keep the seeds submerged.
Add water to the jar and stir it after five days; the good seeds will sink to the bottom. Skim off the pulp, liquid, scum and floating seeds from the top.
Rinse the good seeds in a small sieve. Let the seeds drip-dry in the sieve, then spread them out on a plate or paper towel and let them dry in a place with good air circulation but out of the sun.
Move the seeds around once a day until they're dry. The drying process may take anywhere from three days to two weeks, but err on the side of caution. If the seeds are damp when put into storage, they will mildew.
Store your seeds in a paper envelope. Write the kind of tomato and date harvested or placed in storage. Close the envelope and store it in a glass jar or other airtight container. Place the container in a cool, dark, dry place like the freezer until planting time.