Choose the best tomato on the plant.

Seeds can be saved casually by squeezing them out onto a paper napkin and then air-drying them, but fermentation is a better route.

Fermentation removes germination inhibitors and the gelatinous sheath from seeds, and it may treat some seed-borne diseases. Properly stored tomato seeds may remain viable for over six years.

Fermentation Method: 

Rinse tomatoes in water to remove dirt before harvesting seeds. Squeeze the seeds and juice from the tomato. Do not add water as a substitute for tomato juice since dilution slows fermentation. Set aside the containers for three days at a temperature not more than 70°F (21°C). Stir the fermenting juices to submerge the pulpy material, once or twice daily. This prevents the build up of mold which is not harmful to the seeds but may discolor them. After three days decant. Pour into a larger container that allows you to add three or more times the volume of water and pour off the pulpy water but not the seeds at the bottom. Viable tomato seeds will sink. Repeat two or three times until seeds are clean. Note: not all viable seed varieties sink in water. Then rinse seed under cold running water for seven minutes, constantly agitating and stirring the seed. Sanitize equipment thoroughly between uses to eliminate Late Blight contamination. Pour the seeds into a fine mesh sieve. Lightly spray off remaining gel or debris. If drying the seeds on a screen, spread out with water spray, not your hand. Wet tomato seeds will stick to your hand.Tap the strainer or rub your hand under it to remove excess water. Flip the strainer over, smacking it on a paper plate or paper towel to deposit the seeds or allow seeds to dry on a screen. Let the seeds dry for five to six days at room temperature in a well-ventilated place.
Stir and crumble seeds with your fingers daily to prevent them clumping together. As the seeds dry, lightly rub clumps together between your palms to separate seeds. Store in zip lock plastic bag in a cool, dark, dry place. Do not freeze seeds.