I like to save my tomato seeds each year. These seeds aren't from just any tomato plant bought from the local big chain stores though. I purchased my original seeds from a website that specializes in heirloom and open-pollinated seeds.
So what is heirloom and open pollinated seeds?
The seeds and vegetable plants you buy in the stores are almost always going to be a hybrid. They have been crossed with different varieties. This will create a good crop the first time around. However when you try to plant the seeds from that crop you may or may not get the same result in your second generation. You may get a lot of the variable characteristics that are undesirable.
Open pollinated seeds are like the pure-breds of the plant world. They haven't been crossed and should produce true replicates each year.
Heirloom seeds are an open pollinated variety that is at least fifty years or older. Most are passed down in families.
Here is were I purchased my open pollinated seeds. http://www.heirloomacresseeds.com/default.asp
Growing your open pollinated seeds takes a little care though. You need to be careful that your plants do not cross with other varieties. Tomatoes have been said to not cross pollinate very well due it's retracted style and are mostly self pollinating. There are a few varieties that do however. The currant tomato types and any tomato formed from double blossoms, like the beefstake types, have protruding styles making insect cross pollination possible.
I only grow one type of tomato in my garden so I don't worry about that. And I don't have any neighbors nearby that could be growing different varieties that could contaminate my seeds.
There are tricks to manage this if you do want to grow more than one variety of any type of plant. A great book for growing open pollinated seeds and then saving these seeds is "Seed to Seed" by Suzanne Ashworth.
Here is the way to save tomato seeds:
Start by selecting your best tomatoes from your best plants. You want more of the best right? Cut them open and squeeze out as much of the juice and seeds into a bowl or container.
Let them sit for 2-3 days. This fermentation breaks down the jelly like substance that covers the seeds. The gel contains a chemical that inhibits germination to prevent seed sprouting inside the tomato.
*You may want to keep this outside to ferment. This gets very very smelly! You can't see it well in my picture, but there is a white layer of scum on top.
Rinse off the seeds. I use a small stainer and spoon to mix it around under a stream of water.
Keep going until all of the tomato scum is rinsed off.
Let you seeds dry before you store them. I let mine sit for about 3 days just to be sure. Don't try to dry them on soft cloth or paper or they will become super stuck and hard to remove! I use a coffee filter and it works very well. It helps to wick away moisture and they don't stick.
Be sure to stir them around a couple of times a day to keep them from sticking together.
Package them in a zip bag or sealed jar with the name of your plant variety and year.
Tomato seeds will remain good for 4-10 years depending on your variety and the way you store them. They should be completely dry, sealed in an air tight container and stored in a cool, dry area. They can also be frozen for long term storage.