Here is a tutorial of the process I use, so you can give it a go yourself - it is really easy.
Have you noticed tomato seeds have gel like coating around them? This stops them from germinating immediately when falling on the ground at the end of summer. What we are doing in this seed saving process is naturally removing this "gel" so when you plant them, they will pop right up.
Select the BEST tomato on your plant for seed saving. You only need one, as the seeds in one tomato will be more than enough for you, your neighbours, friends and family!
Scoop the tomato seeds into a jar and add some water. Place a lid on top but don't screw it on.
Leave them for a few days to a week until they get a bit of a mouldy crust on the top. Sometimes this process can smell a little so the gardening shed is a good place.
I have saved quite a few seeds from this year - enough to give a few packets away, so if you would like some just leave me a comment here then drop me an email at firstname.lastname@example.org with your address and I'll send you some. (New Zealand addresses only because of postal restrictions on seeds.)