Collect seed from some of the earliest tomatoes that ripen in the season as these are least likely to carry diseases that may affect the plant later in its life. For a quick and dirty method of saving tomato seeds, smear the seeds onto single layer on a paper towel and allow to dry. The whole paper towel piece can be stored until you wish to use them and individual seeds teared off. A more professional method of saving tomato seeds which reduces the chance of both mould and disease is to first ferment the seeds. Scoop the seeds into a jar and fill with water. Let it ferment for 2-3 days. Pour the seeds into a tea strainer or sieve and use tap water to wash any pulp through the sieve leaving only clean seeds. Tip onto a paper towel to dry before storage. Tomato seeds stored under cool and dry conditions will remain viable for at least 4 years.