One of the things I like to teach people in my classes is that once you learn to save seeds from tomatoes you should never have to buy new seeds again (unless you want to try a new variety).
Choosing to save seeds from the healthiest plants is always an important first step.
Next, remember that tomatoes are in-breeders (or selfers), so you don't need to save from very many plants. They can cross in the field with other tomatoes however. I have personally witnessed bumble bees pollinating my tomatoes. But isolating varieties by planting populations 10 feet or so apart should cut down crossing significantly in an average situation. If you have an incredible diverse and biologically active farm or garden you might want to isolate further.
When saving seed, pick very ripe fruit. Then, follow these easy steps:
1. cut fruit in half
2. scoop seeds and jelly into a jar. Add a tiny bit of water if necessary
3. cover jar loosely with lid or cloth to keep insects out.
4. let sit for a few days in a warm place
5. check every day for signs of fermentation. This would be bubbles forming in liquid, white film forming on top of liquid, a sharp smell
6. once signs of fermentation appear, test seeds to see if the jelly coating is still surrounding seeds. If they are still slippery to the touch, ferment another day or two longer. If seeds feel "hairy" or just not slippery, the jelly has fermented off
7. rinse seeds to separate from jelly
8. once jelly has been rinsed off, pour into colander
9. put on a plate to dry. Make sure this is not a paper plate or paper napkin or you won't ever get them off!
10. put seeds in an envelope and label with year, variety, and location of harvest. Voila!