At the start of the season it is important to clearly label your tomato plants so you know exactly what variety of tomato seeds you are collecting.

Pick three or four tomatoes, preferably from more than one plant of the same variety.  Do not pick any tomatoes from sickly looking plants as they may be diseased.  Many plant diseases can be transmitted through seeds.

While tomato seeds in a tomato are immersed in the juice of the tomato they do not germinate.  This is because they are surrounded by a jelly like substance that stops the germination process.  To prepare the seeds for drying it is useful to break down the jelly like substance that surrounds the seed.

To do this slice the tomatoes in halves or quarters (depending on the size of the tomatoes) and squeeze the seeds and pulp into a glass.  Add 1/2 to 1 teaspoon of water (depending on how many tomato seeds you have), give a short stir with a spoon and leave overnight  The added water will start to break down the protective jelly coating surrounding  the seeds.

Do not leave the seeds in the water mixture for more then twenty four hours as once the protective jelly has completely broken down the seeds will begin to sprout, thereby ruining them.  If this happens then throw the seeds out and start again.

Tip the seed mixture into a sieve and hose with a jet spray. It usually only takes about twenty to thirty seconds of hosing to remove all pulp and jelly from the seeds.  The hosing should preferably be done over a garden bed so as not to waste water.

Immediately tip the seeds onto a sheet of newspaper and spread them out.  Place the sheet in a dry shady spot and leave the seeds to dry out.  They will generally dry within an hour but I usually leave them on the paper for about a day.

Scrape the seeds off the paper into an envelope and store it in a dry shady place that is rodent proof.  Make sure you label the envelope.

It is possible to skip mixing the seeds and pulp with water and leaving overnight in a jar (as outlined in step 2.) and go straight to hosing the seeds in a sieve.  You will just have to hose for  longer to make sure you have removed all the protective jelly surrounding the seeds.

You can also simply squeeze the tomato pulp with the seeds in it directly onto strips of newspaper and smear the mixture thinly over the paper.  Once dry the seeds will have stuck to the paper, which can be stored until required.  Though this method is very easy I prefer the sieve method as storing the seeds on sheets of paper can be awkward.