Around this time of year the rains start falling, the temperature starts to drop and the tomatoes start to decline. So, what to do? Two words, one answer: SEED SAVING.
Here is the seed saving method for tomatoes that will give you the best results:
Cut the tomatoe in half and squeeze the seeds and pulp onto a plate or plastic dish.
Let the seeds sit for a few days. (A mold will develop on them, don't get excited. This is a good thing)
Wash the seeds with warm water and pat dry with paper towel. Allow to dry on the paper towel.
Store in some sort of airtight container (ziplock bag, tupperware, used plastic containers)
Be sure to label your containers.
The other way to save seeds and not have to worry about any sort of storage is to just drop the tomatoes that you really like into the area that you want to grow them next year. You will have a tonne of "freebees" pop up next spring.
Though, with this method; you won't have any seeds to trade with. You will, however, have seedlings to trade with in the spring.