Seed saving why it is important?
Without seeds there's no hope of survival. Seeds is reproductive material- a new life and genetic information stronger plants produce stronger new plants. If people focus on saving seeds and ecology the life on this planet would be happier and healthier. Plants are everything its gives life -produce life- and continues life....With a dangers of GMO (genetically modified organisms)we are loosing the most important thing which is seed heritage when Gmo plants cross pollinates with heirloom it unfortunately they changes them .

Here's tips how to save tomato seeds

Tomatoes

Lycopersicon lycopersicum

Almost all modern tomatoes can be safely grown without isolation and will not cross—'currant' tomatoes (such as Cherry Tomatoes), and 'potato-leafed' tomatoes (such as Brandywine) are possible exceptions and may cross other currant or potato-leaf varieties. Grow as many standard tomatoes as desired, but grow only one currant tomato or one potato-leaf tomato at a time to ensure purity (or cage them, or separate varieties by 500 feet). Currant and potato-leaf tomatoes will not usually cross with common tomato varieties.
It's best to not plant all a valuable variety's seeds in one season until you are sure it doesn't cross with any other varieties you grow.
Allow tomatoes to ripen thoroughly on their vines to at least the eating stage before harvesting them to collect their seeds. Upon harvesting, tomato seeds are best fermented in order to remove a germination-inhibiting gel which covers the seeds, and to kill diseases. In nature, fermentation of fallen ripe fruits removes this gel, and this process is imitated when preparing tomato seeds.
If fermenting tomato seeds seems too much trouble, they will still germinate if the slippery gel surrounding the seeds is carefully rubbed off while you're cleaning them. Seeds treated this way will germinate, but they will not have had the protection of the fermentation process killing disease organisms. If you noticed any problems with your plants (leaves spotting or dying, inexplicable wilting, etc.), the extra trouble of fermentation will be well worth the effort.
Dry your tomato seeds on a piece of glass or a shiny plate—the wet seeds will stick to paper and be difficult to remove without damaging them.
Tomato seeds will store safely for 4 or more years after being properly dried and stored.

How to Ferment Seeds

To prepare seeds for fermenting, simply squeeze or scoop the seeds—together with the pulp that surrounds them—into a jar with a little water (about half as much water as seeds and pulp). There is no need to include more pulp than naturally comes with the seeds. Store this seed/pulp mixture in a warm place (75 to 85º F) for 1½ to 5 days (depending on the seed type and whether conditions are warmer or cooler).
Fermentation will be evidenced by bubbling and/or by the formation of a white mold on the surface of the mixture. As soon as the bubbling or mold have been evident for a day or so, pour the mix into a bowl and clean with running water.
Watch closely, as seeds left fermenting too long (especially above 80º F or so) may germinate, ruining their chances for storage. Once the seeds start to 'imbibe' or swell due to taking on water, they will have begun their internal process of germination... by the time their tiny roots have begun to emerge, it is far too late to try and dry them for storage. Sprouted seeds can be planted immediately and grown out (depending on season), but they will die if they are dried out for storage once they have begun to germinate.
It's not required to ferment squash or eggplant seeds, though it increases their germination rates and kills some seed-borne diseases. In general, when temperatures are kept between 75 and 80º F or so, fermenting is safe and beneficial and will be safely completed before seeds begin the process of germination.

For  more info useful sites how to save your garden seeds  see http://www.seedsave.org/issi/904/beginner.html or http://howtosaveseeds.com/seedsavingdetails.php#tomatoes